My Almost Stepson

I’m not sure where to begin...but I am about to marry a great man. He’s 17 years older than I am. He has an 18 year old son that spends every other weekend. It was totally okay at first. I dressed completely covered all the time. We do typical family things. At our house last weekend after we watched a movie together I noticed my stepson checking me out. I mean drooling at my ass. I was kind of shocked and pretended I didn’t notice. I kind of felt uncomfortable. The next day his dad and I were having sex and all I could think about was my stepson watching me making his dad cum. Idk why or what’s wrong with me! But I came even harder imagining pleasing my fiancé while his kid was watching. Now I’m wondering if his son masturbates to my ass. Am I making both father and son cum? And why is that a turn on for me??? I’m not even attracted to his son, he’s a teenage boy and I love older men.