Medical advances. In the future, science will eliminate all risks of sex except burning yourself on your partner's jet pack exhaust.
Magic. Waving your wand and pronouncing the word "prophylaxis" gives reliable protection from both pregnancy and STDs. Only Muggle-born wizards know what this word means.
Healing factor. I think we're all agreed that Logan can have all the unprotected gay sex he wants to. Het is another story -- self-repairing, functionally immortal sperm could wait *years* for you to go off the Pill.
Vampirism. There is no scientific basis for the myth that vampires or mosquitoes can transmit HIV. Vampire semen cannot fertilize an egg unless invited.
Interspecies sex. Charles and Lilandra have nothing to worry about except bird flu.
Sex with plants. Getting Swamp Thing excited is not recommended if you're prone to hay fever; otherwise, no problem.
Sex with robots. Just clean them well afterward. (Does not apply if you are the Scarlet Witch.)
Luck. Sex with Longshot should be safe unless having a child would be lucky for him in some way, in which case the condom would have broken anyhow. See also Teela Brown.
Precognition. Every few years since Destiny's death, Mystique finds a list of names and future dates taped to a box of condoms in the back of a closet, or delivered by FedEx from a nonexistent address in Myanmar. She consults these lists religiously. Everything else is safe.
That can't be all. What am I missing?