I don't know you or this guy so I'm not going to make any assumptions, but just give my opinion of what you've said so far. There are several possible reason as to why he's doing this to you.
1.) Maybe he genuinely likes you and this is his way of trying to flirt with you? True, he may be a dumbass for using this method, but then again well it would certainly explain why he's specifically choosing that technique.
2.) Some people pick up on other people's personalities, bullies obviously do what they do because they are insecure themselves. When I say this I mean most if not all Type A people can sniff out Type B people. If you're more on the quiet side and clearly don't know how to slingshot a comeback to his remarks, this only encourages the aggressor.
On the issue of teasing him back, well first you have to stop being that quiet timid person. Obviously this doesn't happen overnight and chances are it take awhile for you to change this unless you genuinely want to find out A.)why he's doing this or B.)if he's doing this because he likes you.
While no one can genuinely help you on changing who you are in terms of being reserved I can make some suggestions for returning those quick-wit remarks of his. The next time he makes some sexual jokes, just smile and say return fire with something oriented towards him of the relative equal level.
In other words if he jokes about you 'sleeping around', don't return with "yeah, well you have a small peep and I heard you'll be making live appearance with Chris Hansen soon". Maybe return with something like, "yeah, well at least all my partners don't charge by the hour".
Ok, as for your last part now you have me confused. Do you like this jackass? The first part of your post seemed as though your painting this genius as a bully and asking for advice on how to deal with him, that I can fully understand. But you're entertaining the idea you wouldn't mind these quips if his intentions are that he genuinely likes you?
Well, if you WOULD like him if you find out he was doing this to woo you (in his own retarded method), then you really should ask yourself some questions as to what you want (and not on this website). Most people who don't mind teasing or jokes at their expense wouldn't ask on how to deal with bullies, so that's a no brainer.
But since you seem to harbor some interest for him if he is doing all this for you, well I have can't add anything since that doesn't make any sense to me. You really have to make up your mind, if you LIKE him but hate the personally-aimed insults then write that out so others can add their opinions on how to handle that. If you DON'T like him then feel free to take my suggestions about countering this remarks or ignore them.
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