Looking for a (harem) manga where MC is 100% straight, but there's a bi/gay moment or two. He's definitely not bi, but there's a one-time "indiscretion" with another guy. Like an adult version of Yamada-kun and the Seven Witches but the "bi" stuff happens like once or twice?

Maybe his best friend (GG) is gay and loves him and is always sad MC is getting girls and will never love him back. Maybe for GG's birthday, MC "allows" GG to suck him. Or maybe they do have sex--MC on top, of course. He's straight but just doing it to make GG happy. Kinda like UQ Holder if

Kuromaru chose to be a guy .

In Parallel Paradise, MC is the only guy in the world. Touching makes the girls insta-horny. Sometimes sex happens for "convenience" reasons and he goes along with it. Let's say another guy shows up, but he's gay. Like the girls, MC touching him makes him insta-horny. Maybe because they're on a mission together, MC "allows" GG to suck him. Or maybe they do have sex--MC on top, of course. He's straight but just doing it for the mission. One time MC and three girls were stranded at sea for days, so of course they had nothing better to do than sex. Maybe a panel or two could show GG with MC since they're all stuck there so "why not"?

The items in Sex & Dungeon conveniently only work on the opposite gender, but what if it weren't gender specific? MC might want to use a love potion on a girl he likes, but at the last second a guy inadvertently drinks and wants MC. MC might feel responsible for what happened or goes along with it for convenience reasons, and doesn't feel too conflicted about it because he would still be "the guy" when it happens.

Not talking about traps or straight-turned-bi. He's straight but for whatever has an "indiscretion" with a guy and that's as bi/gay as it gets. Or at most, like UQ Holder, maybe his gay best friend might be the one exception to MC's "girls only" rule.