[Matt10k]

Few things that could be immoral (to me): Person has an STi they know about but makes no effect to protect others from it, person cheats- is not single, has sex with an underage person or with someone who doesn't consent, makes no effort to stop themselves getting pregnant despite the fact they don't want children but more importantly would not be able to properly look after them if they did, impregnates someone and cuts all ties afterwards.

If you avoid that minefield I don't think it's immoral at all but then there are some religions that see sex as wrong or immoral too or those that think sex before marriage or outside a relationship ie one night stands are wrong but I personally don’t subscribe to any of these ideas...