Downplayed: Bob is in love with Alice, but she's so busy saving the world that they don't have the time to form a relationship.
Justified: Alice is serving under a god whose tenets include Sex Is Evil, and Bob must respect that or else everything is ruined.
Inverted: Bob is not only allowed, but encouraged to have sex with Alice before she summons her god.
Subverted: Alice can have a relationship with Bob, and even sex with him.
Double subverted: After the MacGuffin is found and the god is summoned. Until then, Alice must remain a Celibate Hero.
Alice becomes frail and feeble as soon as a man so much as touches her.
Some of this particular god's Mikos are allowed to have sex, others are not.
Alternatively. Bob knows some things are forbidden and aren't fooling around...but some things he just wants more because they're taboo.
Bob knows that when Alice tells him that something bad happens when she loses her virginity, that she's not kidding. So he doesn't do anything.
Alice is not forbidden from having sexual contact before summoning her god.
Enforced: "We need something to spice up the plotline a little..."
"Alice, you must remain a virgin through all your ordeals."
"How come every time something becomes taboo, we all suddenly want it?"
Invoked: Alice agrees to be a Miko, which includes the caveat that she's not to have any sexual contact as long as she is.
Exploited: Alice uses this trope to get Bob to do whatever she wants.
Defied: Alice and Bob know they aren't supposed to have sex, but their urges are too strong, and they do it. (This may or may not result in catastrophe.)
Alice: You must know, we cannot take our relationship to the next level...I will be unable to summon my God if I do so. Bob: How come all the really really attractive girls are off-limits? Alice: I really really shouldn't have told you this...because now you'll want it more.
Reconstructed: Alice and Bob resist the temptation, and the plot points are resolved. Their relationship is stronger and the sex is better when they eventually have it because it's not something they'll be taking for granted.
Played For Laughs:
The forbidden thing in question is not some girl's virginity nor something that dramatic, but... Bob's roommate's last can of beer.
The forbidden fruit is... Alice's fruit parfait. Because she wants it.