Sex is considered a physical manifestation of love by our society. That’s not universal but there’s a strong connection between the two; if there weren’t we wouldn’t call it “making love” (among many other things). I’d say there’s a stronger link between sex and passion than sex and love.
Sex without love can still be good and healthy, but sex without passion is boring. Lust is a kind of passion, a passion for the physical. Love itself is obviously passionate (if you disagree, then you and I have a serious difference on our definition of love). So those are both equally great drives for sex.
Anger or pain can both be passionate and they can both be channeled into sex. There’s nothing wrong with angry sex, it can be a very good outlet (assuming all involved parties are wanting it). Sex as a pain response, after suffering a major loss or trauma, can be therapeutic given the right person and circumstance. These may both be ‘negative’ emotions but that doesn’t discount the power of the feelings they can create.
Saying only happy sex is okay is like saying it’s never okay being happy while alone. We’re all different; how we feel and deal with our emotions is unique.
Just don’t have boring sex, that’s a waste of a wonderful thing.