Here are two examples to give you an idea of when to use each one:

1) Q: How many beers do you want? A: One (Eins)

2) Q: How many beers do you want? A: None (Keins)

Keins is basically the negative form of eins. So, you could say "Ich moechte ein Bier bitte," (I'd like a beer please) or "Ich moechte kein Bier danke." (Thanks, but I don't want a beer.)

Well, actually the grammar to this sentence does not make sense, because Gesclecht means gender or "sex" the direct translation would be "I would like to have sex(as in gender) with you" but what I am assumping the person meant by this is that he or she would like to have sex with you.

I would have said "Ich moechte mit dir sex haben" and I am pretty sure that is grammatically correct or if not, it would be comprehensible in its meaning.