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I was doing laundry, and my girlfriend's panties were inside out and had a large, slightly yellowish, crusty stain on the crotch. We have not been having sex lately. I know that girls often get stains there from mucus and what not, but because this was crusty. I am alarmed that it might be semen backflow from another man. Are girls' mucus stains ever crusty? If there is semen there, should it smell like it even though it is dried? Please tell me anything you know.
No, panty crust is a normal result of vaginal discharge. Normally, it's just cervical mucus. If she has an infection, it may contain other things (like pus). Cervical mucus -- like all mucus -- dries out to a solid; there's not much of it left compared to its original volume since mucus is mostly water. If the site is healthy and free of infections, the mucus will dry up yellowish. If there's something wrong (like throat and nasal mucus) it'll look greenish, cloudy or otherwise discolored. As for scents, dry mucus will smell very faintly vinegary (like fresh mucus, but faint). Cervical mucus and vaginal discharges will vary in consistency and volume depending on her cycle. If she is fertile, she'll generally have a more profuse, fluid discharge. If she's not optimally fertile, she should be secreting a pasty, sticky mucus.
Whatever the case, mucus (healthy or not) is water-soluble and washes out quite easily with detergent. I'd also recommend using a product like OxyClean to whiten up the yellowed patch in the crotch. It boosts stain removal without damaging the dyes in your clothes.
If I were you, I wouldn't worry about another man's backflow. But if you have some real reason to suspect something's up, you may want to re-evaluate the state of your relationship. Otherwise, you may want to re-evaluate how you perceive your partner and the relationship (i.e., any underlying trust issues?).
For more info, check out:Honorably Discharged: A Guide to Vaginal Secretions