Does sex relieve stress?  If so, should people have sex more often?

Yes, sex can relieve stress, but only if the all individuals involved wish to have sex for that reason.

My semi-educated guess is that most people that want to have sex (and actually have sex) do so because it’s highly pleasurable and fulfilling for themselves and/or for their partner(s). In other words …

Sex = (Mutual) Pleasure …

And a byproduct of that may often be stress relief, better sleep, emotional and physical closeness, excitement, the challenge of trying something new, comfort, feeling “bad” in a “good” way, feeling “good” in a “bad” way, and all the other myriad reasons why people have sex and seek to have sex.

However, I would caution against seeking to have sex and having sex as a way to fix or alleviate a stressful situation. The goal in this situation is to alleviate or fix the stressful situation apart from sex. If you start using your partners sexually for relief of another problem, they’re going to resent the manipulation. In other words, …

Sex ≠ Stress Relief.

The correct equation in this particular instance is …

Dealing Directly With The Stressful Situation = Stress Relief.

Furthermore, trying to convince someone of the health benefits of sex who [a] doesn’t want to have sex with you or [b] doesn’t want to have sex with you right now will most likely result in increased stress for them. Respect their choices, even if those choices include not having sex with you. Instead, wait until you address the problem preventing everyone having satisfying sex, have sex with yourself, have sex with someone else not them, or have sex with other people besides them. Stress should be relieved and avoided all around for everyone if you respect everyone’s consent.