Inspired by this thread, I was just wondering…I haven’t a clue, being neither gay nor a guy.
Get married
Occupy the White House
Public displays of affection outside of gay bars or well lit gay ghettos.
Have good relationships with both sets of parents (it happens but it’s very rare).
Buy off the rack at K-Mart (it’s just not in us).
Talk about their dates last weekend or celebrity asses they like in a primarily straight gym.
I’ve been powerlifting and bodybuilding for over twenty years. I’ve yet to find a primarily straight gym that has modern equipment.
Pick out curtains? 
Yeah, I prefer to go curtain-picking-out with my lesbian.
Wait…
Are you asking what they are unable to do or what they are able to not do?
Reproduce?
Have hot lesbian sex.
From these threads, it seems like there are a lot more things straight guys can’t do than gay guys. Maybe they’re onto something. 
Darn tootin’. Why do you think I signed up?
Wouldn’t the corresponding thread be “some things a gay guy cannot do with a female friend”?
(Actually I can’t think of anything either.)
Go shopping for firearms
One word: NASCAR.
Actually most of the gay guys I know (including myself) are pretty well armed and crackerjack shots. Not surprising as about half are ex-military.
Lamaze?
Experience an unplanned pregnancy?
Vaginal intercourse.
Eww.