Are you sexually attracted to them in the first place? Or just romantically? I'm thinking maybe on the asexual spectrum - it appears possible to be romantically interested, but not sexually, or less sexually (it's not necessarily on/off, more often a graded scale.
Could the scenario be that you're romantically attracted to them, have some degree of sexual attraction, but the act of sex itself is not attractive to you (hence the asexuality) and then afterwards you're only able to associate them with the sex which you hadn't really enjoyed?
If it's asexuality then there's nothing 'wrong' with you, but you might want to think about whether you really want to have sex, and consider a dating site with other asexual people and find someone compatible. Consider also whether it's all types of sex that cause the ick, or just penetrative. Some asexual people will happily engage in sensual type activity but avoid penetrative sex, for example. I'm not asexual but I've read around the subject.
Just a thought.
Otherwise I'm thinking maybe some deep-seated memory of sex being called dirty (a parent's attitude perhaps), and for some reason you've turned that feeling onto other people instead of, as is more common, yourself. If this seems more like it, then I'd suggest counselling.