Open relationships are the most fun when the couple play together. IE bring a girl or a boy home and just have fun all 3 of you It's not odd at all, though. Don't let some puritan judge you. You're harming noone and pleasing a number of people. You rock.
I got married when I was 18 and its been nothing but great. Any marriage no matter how old u are has its ups and downs but you make it through it. My husband and I also have an open relationship and it is just what works for it. its actually more common then what people think. I actually am a member of 2 websites (swinger sites) and was very shocked to see how many people in my own town with that worked with us and such that were the same way.
Your biggest mistake was not locking the door, dude! Your relationship with his daughter would be weird for me, but so what? It's YOUR relationship, so it is not weird for you. It is for her father, but the poor guy probably doesn't know she's bi and been nooky snoopin' as well as sexing you. Do him good to know the truth. She should be honest with her dad; would solve the problem right away.
QFT People of our generation aren't so quick to define sexuality or relationships...good on ya for being so open and trusting of one another!