Is it normal that my fiance has white stains in her pants?

If you get off thinking that it's dried semen in her shorts, go for it. However, it's entirely possible she's being honest when she says she isn't having sex with another guy.

The quantity and nature of vaginal lubrication changes during the monthly cycle. In fact, it's possible to tell by the stickiness of vaginal secretions where the woman is in her ovulation cycle.

I'm sure you also know that vaginal secretions increase with arousal. What a lot of women don't know is that vaginal lubrication can be triggered by lots of things that they don't consciously think of as sexy. (A famous study found that watching film of Bonobo chimps mating increased vaginal secretions. That doesn't mean the women wanted to have sex with the chimps, just that the vagina responded when the women were presented with a sexual situation.)

If you're really (obsessively) interested in this, you could try to correlate the amount of white stains with her period for a few months. You could even try an experiment and try to get her turned-on on a day when she shouldn't have much staining - prolonged sexy phone or text teasing, or something like that.