Whenever you're looking for scientific studies or papers, start your search at https://scholar.google.com/. This is a specialized branch of the Google search engine that searches for scientific papers, preprints (scientific papers that have not gone through peer review), books and science magazine articles.
Searching for "male canine aggression" yields a plethora of results, most pointing towards:
Short answer: Intact male dogs are more likely to be aggressive than neutered male and intact or spayed female dogs. I could not find much information about the frequency of aggression towards intact or neutered dogs. Inter-dog aggression can be reduced by educating dog parents and training the dogs.
Long answer:
The publications most commonly base their results on reports of aggression by the owner of dogs, medical (veterinary) care providers, shelters or animal welfare organizations.
Furthermore, the reports and studies focus much more on the aggressor, less on the victim. Most studies categorize the victims of dog aggression as either familiar or unfamiliar humans, and familiar or unfamiliar dogs, but don't further distinguish between the sex or reproductive status of attacked dogs.
Studies showing that male dogs are more likely to be aggressive:
Clinical classification of canine aggression (published in 1983):
Complete case histories from 120 dogs referred because of aggression were reviewed[...]. Sixty percent of the animals were male, with the remaining 40% divided almost equally between females, neutered males, and neutered females. [...] Attacks or threats were directed to adults by 54.2% of the dogs, toward children by 21.7% of the dogs, and toward other dogs by 41.7% of the patients. Twenty animals (16.7%) threatened more than one category. Competitive (dominance) aggression, the most frequent type diagnosed, was expressed by 59.2% of the dogs and was generally responsive to treatment.
The book The Domestic Dog: Its Evolution, Behavior and Interactions with People has an extensive overview of several studies, but it's displayed as an image, so I cannot copy any text from it. In summary: Several studies found that male dogs are more aggressive towards humans and other dogs. Several other studies found that castration and spaying increases aggression in both male and female dogs.
Analysis of 1040 cases of canine aggression in a referral practice in Spain (published in 2007) summarizes:
Animal behaviorists’ databases consistently show that male dogs are more frequently presented for aggression (Beaver 1999, APBC Annual Review of Cases 2005, Bamberger and Houpt 2006). Although the influence of sexual hormones in the arousal component of aggression is still controversial, neutering is currently recommended in many countries as a humane way to avoid unwanted reproduction and also to prevent behavior problems (Borchelt 1983, Podberscek and Serpell 1996). However, the percentage of dogs neutered can vary within and across countries, and so influence the prevalence of different behavior problems. For instance, the rate of neutered dogs in the APBC annual report of cases has increased from 36% in 1996 to 62.6% in 2005 (APBC Annual Review of Cases 1996, APBC Annual Review of Cases 2005).
Studies showing a connection between inter-male aggression and possessive behavior
Canine dominance-associated aggression: concepts, incidence, and treatment in a private behavior practice (published in 1997) writes:
A review of 35 DA cases presented to a small, private, behavior-only veterinary practice revealed that attention addiction was the most commonly associated (66%) secondary diagnosis. The diagnosis of DA was based on standard criteria. Treatments emphasized owner education and understanding of the problem in addition to common behavior modification, surgical, and pharmacological therapies. The necessity for the owners' gaining psychological leadership in relation to the dog was central to the suggested therapy for 34 of the 35 cases. [...] Owners' reports showed that 12% of their dogs showed excellent improvement, 44% reported good improvement, and 32% fair improvement. [...]
Aggressive behavior of dogs kept as companion animals: Classification and influence of sex, reproductive status and breed (published in 1983):
Data were obtained from a total of 245 cases of aggressive-behavior problems in dogs kept as companion animals. Aggression involved barking, growling and biting behavior. [...] Eight major types of aggression were observed: aggression related to fear; dominance; possessiveness; protectiveness; predation; punishment; pain; and intra-specific aggression.
[...] More aggression problems occurred in males than females, with dominance and inter-male aggression showing the greatest likelihood of occurrence in males. [...] Tabulation of associations among these problems revealed that dominance and possessive aggression frequently occurred together, fear-elicited and intra-specific aggression frequently occurred alone, and many of the aggression problems were associated with non-aggressive problems related to fear (phobias) and anxiety.
Attention addiction or possessive aggression have nothing to do with reproduction, ans are therefore probably not influenced by the sex of their target. This implies (but doesn't prove) that dogs exhibiting aggression towards other dogs don't care about the sex of their target.
Studies connecting inter-dog aggression with learned behaviors:
Management and rehabilitation of inter-dog aggression in animal shelters has a very long and thorough list of cited studies, including:
Intact males are not only the most common aggresors in dog fights, but also the most common targets (Roll and Unshelm, 1997).
A dog's reactivity to other dogs can be influenced by its experiences during early life [...]. Indeed, Roll and Unshelm (1997) found that almost half of aggressors and victims of dog attacks had few interactions with other dogs between 5 weeks and 5 months of age. Many authors therefore emphasize the need for prevention and early diagnosis of inter-dog aggression.
A note on the effectiveness of behavioural rehabilitation for reducing inter-dog aggression in shelter dogs (published in 2008):
The effectiveness of a rehabilitation program for reducing inter-dog aggression was evaluated at the municipal animal shelter. Sixteen dogs (of 60 examined) met the study criteria of medium inter-dog aggression as determined by an inter-dog aggression test. These dogs received a 10-day treatment of daily rehabilitation for 30 min (rehabilitation group, n = 9) or daily release into an outdoor enclosure for 30 min (control group, n = 7). Rehabilitation consisted of desensitising and counter-conditioning dogs to the approach of other “stimulus” dogs. Most dogs in the rehabilitation group showed a decline in aggression scores when re-tested after the last treatment (day 11), and differed significantly from the control dogs which showed either an increase or no change in aggression scores (U = 8.5, P < 0.01). [...] The differences between groups were no longer significant when a reduced sample of dogs was tested 1 week after rehabilitation ended (day 18). The study shows short-term reduction of inter-dog aggression through rehabilitation, but further work is needed on effective ways of maintaining the behavioural change.
Again, if training or therapy are effective measures against aggression, the sex of the target is irrelevant.
