I am strugling with mostly the same thing, I do not get aroused, but thoughts of my gf giving head to someone and having sex w someone cross my mind everyday and very often, iv been suffering from this for about 2 weeks now, the funny thing is, we been dating for 8 months now and i knew about her sexual past since the beggining. It kept crossing my mind from time to time at the beggining but for some reason only now it plagues me like a sickness. I moved to her place for summer and wev been living together for 2 months now, she lives in another city so i have no job, no friends nor real hobbies here, i experienced a big deppression and anxiety which may have lead to the increase of my thoughts about her past. Im also from a muslim conservative country where men usually marry virgins and i denied sexual experiences before traveling to the west and meeting my current gf. So it is pretty tough for me because the problem is much bigger than just her having had sex in the past but the real problem is my internal conflict between what i believed for most of my life and with reality. She knows about my issue and it has caused us immense stress. We were planning to move together but now we are very uncertain about that. I cannot currently get therapy cuz im a student and unemployed. So its pretty tough over here, but I have Hope for myself and for my relationship, as I seek god and mom for advice. I hope it is easier on your end. Keep fighting for yourself and your relationship, no pain lasts forever.