I used to do this years ago but it was never sexual, just cute profiles and pictures and never really with people I knew. I examined why I did it and I think it was a sort of "love porn" where I would imagine what it would be like to have them in my life. I never actually went back and looked at them though, I just felt like I needed to save the photos. I never tried to justify it and eventually realized it served no purpose and deleted them all. Ultimately it was innocent but I can see why people would think it was weird because I wasn't getting off to them so "Why?". Hard to explain but I've seen examples of others doing it, it's not super uncommon.