When my husband told his mother about our gf, via email, she promptly called him and said, "So, Kari's a lesbian?" He said, "no, she's bisexual." She then said, "I thought you and Kari were supposedly so happy. So, why this?" He was pissed, and hung up on her (it is worth noting that they have a tumultuous relationship at best - and he's always believed that he'd cut her off at some point after his father died, which happened, sadly, last fall).
She then replied to his email with an email so hurtful and vicious, he wouldn't even allow me to read all of it. What he did sum up was that she said, obviously he's miserable with me and so she'd help him buy his way out of the marriage. She also told him his father wouldn't be proud of him because of this (so not true - his father was only interested in his son's happiness).
Needless to say, this was the straw that broke the camel's back. She gets her sister to call and leave guilt trip messages all the time. It's horrible, and my heart breaks for him because he said to her in the email that he wanted to be able to tell her about this because it was something major for him, made him happy and after all the hardship of the last year (with his dad dying), it was such a nice thing to have that sort of happiness. She didn't have to agree with him, but did she have to attack our marriage (she's always hated me, btw)?
With my parents, it was easier. My mother and I also don't have a great relationship, and there was much drama last summer that resulted in me basically refusing to share ANY of my personal life with her. So, basically I said to her, look, this is something I am going to share with you, after all that's happened, I hope you'll know better than to criticize me. She, unlike my mother-in-law, is not prepared to jeopardize my talking to her (she needs me for various reasons), and so she bit her tongue. It was done loudly... meaning I could hear "concern" in her tone, but she has never criticized our decision.
My father is a former hippie. He's happy if I'm happy. My relationship with him isn't easy, but he's also not one to judge this sort of thing. If it works for me, it's fine by him.
My teenage sister says that if I'm lucky enough to have two people love me this way, she's happy for me, and maybe a bit jealous (especially now that she's pregnant and the baby's father is an ass - to be nice about it). She actually asked me questions that surprised me... things about jealousy and the like. But she was extremely accepting and mature, and can't wait to meet our gf (she lives across the country, so it may be a while).
As for friends, most have the "it wouldn't work for me, but if it makes you happy, good for you" mentality. A few have pulled away from me a bit, but not actually criticized or condemned the relationship, and only one had the sort of attitude similar to my mother-in-law, but in a much nicer way.