What about serious sex then? Sex for love?
Sex for love is also sex for fun. Only difference is how committed you are to the other person and to continuing your relationship with them. "No strings attached" arrangements would be fifth house, for sure, but sex in the context of a marriage or a deeply committed relationship would still have the fifth house element of enjoyment. (And some people see non-committed sex as still being a form of love. I think you're actually talking about commitment here, committed love?)
I think all sexual acts have both fifth and eighth house in them. The eighth might be looked at more as the underpinnings of a committed partnership. The seventh house is the partnership as it's defined legally and recognized socially: these people are husband and wife, girlfriend and boyfriend, etc. The eighth house is the unseen, more secret side of it: the emotional dynamics, the sexual part of the relationship, the interpersonal chemistry. Also the collective resources: merged finances, things like that.
The eighth house is also the house of taboos. In that respect, prostitution could be considered eighth house, because it's taboo, and usually exists in the shadows. We're not talking about committed relationships in this case, but about the shadow side of sex.