Let's don't confuse hookups (free sex) with Gay-For-Pay pornstars and escorts. Many of them have said it's easier for them to bottom for a guy, there have been rumors about SC and CF using special numbing creams for them not feel much down there while they're getting fucked. I'm not a Doctor, don't get mad if I err on details... ?
I'm NOT saying most bi, bi curious or straightish guys are tops but when money gets involved some folks might do things they won't do for free/fun.