Redefine Your Idea of Sex
Straight couples, there's more to sex than penis-in-vagina intercourse. Vibrators can be used by both men and women, and oral and manual stimulation are other options. If one of you is having a tougher time with libido or pain issues, you can try masturbation together -- even if one of you is just being held while pleasuring yourself or whispering erotic fantasies in your ear, it's a great way to feel close.
Masturbation is, of course, a fabulous option for the single, and LGBT couples can benefit from the same tips.
For people whose libido is seriously compromised by medications or illness, it might be enough to simply have non-sexual physical contact. Holding hands, cuddling, massage and good, old-fashioned making out are just as intimate as intercourse, and there is no shame in acknowledging that sometimes your sex drive is different from your partner's, or that you're having trouble feeling sexual desire at all.