My most recent ex was obsessed about me dating someone else. He harrassed a friend of mine on the old MSN messenger (this was a while ago) to ask her if I was dating. Once my now husband and I went away for a weekend after we'd been dating a bit. A neighbor was taking care of the pets and he (supposedly) was on our street to do a haircut for another neighbor's dog. He saw our neighbor in the yard with the dogs and started interrogating HER about where I was, who I was with. She said all she knew was that I was in Talca, didn't say who I went with to not cause problems. Then, when I got home, there were probably 10 missed calls on the caller ID from his work number.

Fortunately, it died down after a while. From what I heard, he started dating someone with kids and it stopped soon after - one of the main reasons for us breaking up as differing opinions on having children.

It is terrible to deal with, though. If you are no longer with someone it shouldn't be any business of theirs who you are seeing or kissing or whatever. Hopefully he will lose interest in the subject soon.