I agree with some of the other moms that this may be post partum depression, at least in part. I did not experience PPD, but I was definitelty overwhelmed when our first baby arrived and I too lost interest in sex (it had never been a problem before the baby was born). Hormones do incredible things to you. Hang in there a bit longer; once the baby gets older and you are getting more sleep, you will feel a bit refreshed. I did lose contact with friends as well. Get out with them as soon as you can. Go for a manicure or massage and spend some "me" time.
I think that fantasizing about other men or daydreaming about an attractive stranger at the store is pretty normal. We all do that at times. When things are not going well, I think many people wonder how their lives might have been different (dare I say better) if they had chosen another life partner. The fact that you still laugh and joke with your husband means there's still something between you. And I would also venture to guess that the lives of the exes you did not end up with are not as rosy as you imagine. In the end, this is your decision, but I would not give up until you talk to a professional or at the very least, stop being so hard on yourself and schedule a little selfish time. A small part of the problem may be that your life seems like it's no longer your own. Good luck, you can get through this and start to feel better soon.