Well, this is getting a bit convoluted. Hm...
See, I agree that Lelouch isn't pursuing a girlfriend. However, I think the main reason that Lelouch isn't pursuing a girlfriend is because he's too busy pursuing other goals. What you guys seem to be saying is that Lelouch might want a girlfriend, but is holding himself back. Furthermore, you guys are saying that the reason he is holding himself back isn't even because of his circumstances (i.e. it'd be too dangerous for her to be involved with him), but because he doesn't believe in making himself happy. That's just...a bit wrong, unhealthy, depressing, unrealistic, whatever. Something like that. You know though, now that I think about it, I actually have a quote that proves you guys wrong. It's from C.C.'s epilogue, so I hope you guys can believe it.
"People...Humans...chase after the existence of happiness. What the Britannian youth, Lelouch, wanted...was just a tiny bit of happiness. It's nothing special. At the very least, the spark that triggered his actions, was just a very tiny wish that every human has."
Also, the thing with Lelouch's self-sacrifice is that he doesn't go out looking for sins and burdens. Even though he may take up any that he comes across, Lelouch isn't trying to deliberately punish himself (he also isn't trying to save the world--at the very least, not at the micro level). That is why, once he's finished with everything and is in a less sinful/burdened environment, Lelouch
will have room to pursue his own happiness.
I also just have to point out about Lelouch's revenge, as well. Even if revenge doesn't really equate to 'one's own happiness', it's pretty far removed from the idea of self-sacrifice as well.
On Lelouch's charisma (let's just call it 'animal magnetism' to underline the romantic implications), I agree that he doesn't use it and may not even be aware he has it. Still, the reason I brought it up was to say that
if Lelouch decided to pursue somebody romantically by revealing this side of him to them, it wouldn't really be manipulation.
And finally, on your evaluation of Nunally. Actually, the fact that Lelouch still maintains his relationship with her even though she gets sad and misses him backs up my point. If Lelouch truly believed his relationship with Nunally was harmful/painful to her, and if Lelouch was truly an entirely self-sacrificial person, he would give her up. If Lelouch actually thought that Nunally would be happier if she didn't miss him all the time, he would try to think up a method to extract himself from her life and then just watch over her from afar. The fact that he doesn't shows that either Lelouch believes that his love for her is enough to cause a net increase in her happiness, or that Lelouch has a little bit of selfishness within him that will choose his own happiness over someone else's.