Ok here's my experience of something very similar. Right or wrong I'll get it off my chest.
At secondary school, I didn't have many friends. I was bullied for being fat. Eventually, I pretty much had a breakdown. Lost a lot of weight, dropped out, then went to college.
As a 17 year old guy, I started an engineering course. I hoped to make new friends. I hoped to get a girlfriend. Oh, how wrong I was. To say the least, my class peers were not the kindest of people - if there was a certain race their stereotypically doesn't mix well with others, in addition to being loud and boisterous, they predominately made up half the class. The remaining 30% was made up of chavs, and the other 20% were a few smart or geeky kids that kept their heads down.
Long story short there was an ugly girls (one of only 3 girls in the class, the other two were grown women sent on the courses from work) with asperges (high functioning autism) who was being bullied. I stood up for her one time, and she latched on to me. Because of this, people started to dissociate themselves from me also. Pretty soon I found myself spending lunch times alone - only to later be joined by "her".
So without really meaning too, I offered her friendship. Along with this, came a lot of trust. Potentially I was also thinking "hmm, I'm friends with a girl, she sounds kind of sociable (or maybe a liar) maybe I can hook up with her friends."
I'm not sure exactly what happened. We were at lunch, she started crying over something, she asked for a hug, we ended up sitting and hugging. Pretty soon we started kissing. We went back to her house - it was occupied, we went to a park (classy right?) and ended up performing oral sex on each other. Thank god I didn't actually have intercourse with her, even though she offered and claimed to be on the pill, therefore saying I didn't need a condom, thank fuck couldn't get it up due to a mixture of anxiety and complete and utter disgust and distrust.
I can only imagine things being much worse if I had gone through with it - she would likely have trapped me in to pregnancy or such. This girl was bat shit crazy and socially inempt and ugly. I'm honestly digusted and doing anything to her, but I was just curious at the time.
How did I end you may ask?
I simply never went back to college. I went sick a few days before confessing to my parents that I hated it there. I didn't enjoy the class anyway. I dropped out, went to a different college, studys something else for 2 years. Got a girlfriend.
... Then something weird happened.
One day. I'm with my girlfriend of 18 months and her family on a busy pub in a well known location on a saturday morning. Suddenly I look inside the pub (we were sitting outside) and I fucking see her sitting at a table, with 4 girls. It actually makes sense because I recall her wanting to go to the university in this area after college. I find an excuse to leave claiming to be feeling unwell. She looked exactly the same and fortunately didn't recognise me.
TLDR : I was a looser who almost fucked a mentally challenged girl out of pity.