You First
All Luke had to do was come after Justin. How difficult could that possibly be?
There was only one thing that stopped sex with Justin being completely perfect for Luke. The timing. That wasn't so perfect. In fact, the timing really sucked - and not in the fun, fellatio related way. Justin was five years younger than Luke and relatively inexperienced. Luke had enjoyed more lovers than he could count or remember - he knew he was good at sex. There was no good reason why Justin should outlast him every time they hooked up. All Luke had to do was come second, set his mind at ease, and everything would be perfect between them. Hell, if he could do that, he might even consider the serious relationship idea Justin seemed so taken with. How difficult could that possibly be?
Time To Do
The words 'have sex with a man' can change everything, especially when they're written on a straight man's to do list.
Rigby and Brennan have been best friends forever. Growing up and going away to university didn't alter that. Brennan coming out of the closet to his straight best friend didn't change anything either. But when Brennan finds an old to do list mixed in with his friend's possessions, and sees 'have sex with a man' written right there at the top, it alters everything. When Brennan realises the man he's in love with isn't as straight as he'd always thought he was, their friendship changes beyond all recognition. When the friends decide it's time to start ticking off all the items on Rigby's gay to do list, they soon realise nothing will ever be the same - for either of them.