I can relate to what you are feeling. I have obsessed over 2 ex's of my FI's... The first is whom he had his daughter with and she is completely different from me, kinda like a skinny tree huggin hippy with long blonde hair that believes in breastfeeding till like age 5(no offense to anyone if this applies). She is married to a writer/english professor and thinks that she is better than us. She is so different from me and FI. Then there is his ex-wife Suzie, I had the treat of meeting her about a year ago, she is looks like a meth addict with tattoos covering every inch of her body (don't get me wrong, I love tattoos) and she was so skinny, probably from whatever drugs she was taking. FI says she wasn't this bad when he was with her, but come on!
Anyways, it bothers me that my FI was with such different people like this. I ask myself, well he liked "that" at one point in time, how can he like me? We are like night and day! I am a curvy girl, so does that mean he likes these super skinny women, how can he like me? But I don't think about it as much as I use to, Just like you and your FI, my FI and I have talked about it, he is always understanding and I know that he loves me and we were meant to be. His family hated his ex's and his best friend hated them too and they all love me!
By the way, it still doesn't mean that I don't think about it from time to time, I confess that I occationally check the first ex's blog- she's such a nut that sometimes it is good for a laugh!