To answer the last question--I never find myself feeling sorry for single friends, unless they're dying
to be in relationships. I'm actually happier that my friends are single, because I know it gives them more time to focus on themselves, and mentally/emotionally/physically/financially/spiritually prepare themselves for any possible relationships in the near future.
I find myself feeling, moreso, sorry for the friends I know who are boo'ed up. Maybe it's because I'm hearing nothing but whining, nagging, crying, complaining from them about the S/O. In any event, their relationship sounds like more work than anything, and it always seems like they're "all alone in love".
I always feel awkward around my friend's S/O's, especially if there's a dirty joke in the air. I've had 3 friend's men flirt hard with me in the past--1 of them asked my friend if she would consider asking me to join them in the bedroom...
She actually had the nerve to come back and ask me, and I was like, "welp, that's all the time we have for this friendship..."