I have a theory that in many ways a true bromance takes the place of a straight man's need for romantic, monogamous relationships. Let me endeavor to explain...
Many Bromances have a lot of physical touching
Physically touching someone even if it's just a shoulder is a powerful bonding technique that makes someone feel more connected to you. Bros fight, bros give a firm shoulder pat, bros hug it out. In this way, a straight man gets out his need to be touchey feely with another human being just for bonding purposes so when he gets home to his girl he's ready to go straight to the business and rolls his eyes when she insists on a cuddling first.
Many Bromances have a level of emotional intimacy that one or both bros are incapable of having with a woman
Bros, share everything besides a bed. Heck, I know a lot of bros who've slept with one girl at the same time. A Bro can confide in his bro in just about anything. He can lay on his troubles and although his bro won't hold him, his bro will give him hard, true advice that comes from a place of caring and bro love.
Many Bromances are more important to each bro than any girl they could possibly date
They both have a lot of girlfriends they they run through in succession, yet their bromances remains stable and true. They have more fun when they're with each other than when they're apart unless they're having sex or asleep. This is because their bro is the only person with which they can truly be themselves. They've bonded completely with this person and so they experience an endorphin release similar just by seeing them similar to if they ever found their soulmate, which they won't because they're too busy playing foosball with their bro.
If this is making you a little uncomfortable than good, I've proven my point. There's a bit of homosexuality to a bromance that we all pretend is there, but it is. Just like how the huge plot twist ending of Y Tu Mama Tambien that launched the career of one of Mexico's biggest director's (Alfonso Cuarón) when the two players chasing an older woman end up making out and having sex with each other was shocking but pretty believable. I don't think many bros would actually try sex with one another, but in the sense that all gender identity is essentially fluid...they'd probably want to. Sorry Bros...that's just what it seems like...