It may not be exclusively an Asperger's thing.
Interest in attractive people (usually of opposite sex) is very normal.But usually people control themselves and don't act on it.
In this case he's acting on it. It's a little crossing the line already but I don't know your situation. It depends on you, him, your relationship and the culture/environment you live in.
Anyway, if you want him to stop there are a few things. Someone goes against their urges for a number of reasons. You may try all of them.
1. Fear : What will be the consequence if he doesn't stop? Think of something (maybe exaggerate a little) and let him know about it well and clear.
2. Reward : What will he get if he stop? Let him know what he will gain.
3. Fear and rewards : Cut some supplies. I don't know what but you should decide. Let him know what'll be gone and what is it that he's already getting.
4. Empathy : This works on many people in some cases and on some people in many cases. In any case, let him know your pain (not anger!). Let him know how bad it hurts. Only words may not be enough. Give him something to compare with. For example, if you stop going out or stop talking or stop eating (I'm just saying, be wise about it), it will make him know that this pain is lesser than what you are going through. You are willing to go through this pain to avoid some bigger pain. Maybe he'll understand the intensity of that pain and empathize.
Repetition. Sometimes it takes time and repetitions to convey the correct message. Hang in there.
In the end, he's an individual. You can't control him beyond a point. In that case, everything may fail and it'll be hard but wise to accept your fate. People loose things and people go through pain. You won't be the first one and your pain won't be the biggest. In time you'll heal. But I wish such a condition doesn't arise.
I hope I've been of some help. Good luck!