for the most part no, but it depends on how bad it is. I have stretch marks on my arms and legs because before puberty I was so skinny and than gained muscle/fat really fast. I rarely notice the ones I have unless I'm doing something and I happen to be looking right at them.
I would hope not. I personally wouldn't care, if the girl is an amazing person, then why would that even be an issue. If the weight was just recently lost, and you care only because you don't like them, then just do some light cardio/weights, you'll tone up the skin pretty quick and before you know it they will be gone. I had a bad weight problem of losing/gaining very rapidly, so I would get them too, but they disappear in a few months.
Almost all women have stretch marks. There is no avoiding them. If a guy is going to make a big deal out of it then he will forever be alone. My 90 pound cousin has stretch marks. She's never been over 100 pounds. Everyone gets them.