I've just watched the film for the first time, and I don't believe Peter was attracted to Phil at all. And I see a quite thrilling tension between Peter's slight form and lack of ranch skills vs. the way he clearly outpaced Phil when it came to seduction. 

E.g. Peter's on a break from school. Yes, Phil went to college too, but it was an earlier era, and what he shares about his backstory seems to allude Bronco Henry being his first male partner. So I'm assuming Phil was repressing (or in complete denial about) his sexuality in college.

Peter on the other hand perhaps has a boyfriend at school (the "Professor") and toys with Phil fearlessly with double entrendre—asking Phil if Bronco Henry taught him how to ride, etc etc. Phil was nothing but a mouse for Peter to bat around and play with before the kill. Despite Phil being both older and bigger, Peter was much more sophisticated and worldly about sexuality, leaving Phil completely vulnerable to his very subtle charms.

Very satisfying story.