This is a really good question. The few times I have bottomed have always been with guys who identified/said they straight. There was one time where a guy's girlfriend/wife called while we were in the middle of business. He put his hand over my mouth to stop me from making any noises while he took the call, all without missing a beat! I felt bad and it was a buzz kill, but this guy was easily the most attractive person (man or woman) I've ever hooked up with and there was nothing I could do either way, so...yeah.
My opinion aligns with what some of the others have pointed out on here, but I'll also add that since there is the general assumption that men are less picky, less gossipy, and less emotionally attached than women, straight men can satisfy their sexual needs without (as much) rejection and drama. Also, since it is assumed that men are more sexual than women, they can easily find another man rather than having to go through date-like scenarios, discussions, etc., with another woman. A guy approaching a woman for sex is different than a (straight) guy approaching a (gay) guy for sex. With apps, they can be completely discreet beyond sending pics. We all have to admit that for many gay/bi men, sleeping with a straight man is a fantasy in itself. Therefore, straight men (generally) don't have to worry as much about being rejected over their looks and (generally) don't have to worry about another man stalking or threatening him once the deed is done. Both guys go on about their respective business with no emotional strings attached.
Lastly, however flawed or erroneous it may be, I also think that a lot of straight men categorize these flings as something other than gay hookups or cheating. Since there is no emotion or intimacy involved, it is simply them fulfilling their base sexual needs with another person.