Why should I wait until I’m married to have sex if I don’t value romantic love or the institution of marriage?

You don’t have to.

If you don’t want to get married but you want to have sex, it is your responsibility to (1) find like-minded individuals that want what you want, (2) make sure all involved individuals are actually interested in each other and consenting to the sex being offered, and (3) everyone is protected against STIs and/or unwanted pregnancy.

You will face resistance generally (not necessarily specifically—even though that can happen) because most societies are geared around the “one true relationship” model.

Another one of your responsibilities is to know yourself, your desires, and your interests to such a thorough extent that you can define with ease precisely who you are and what you are about to any potential partners.

People who don’t want what you want should be filtered out of your dating pool. Then practice great consent skills with the remainder you fancy and that fancy you back.

All the best.