Equality was the agenda; you're right, it's very liberal, very progressive. Even if at the time the founders though only white men who owned property deserved equality, it was a very enlightened principle.
And bit by bit, we're making their dreams come true in ways they never could imagine.
Yes, marriage equality is true in spirit to our founding documents.
In terms of abortion - being able to control one's body is a pretty fundamental freedom. Of course, in 1776 women weren't considered full citizens, so founders had no trouble controlling their bodies; again, bit by bit we are making our country be more true to its ideals. But as it turned out, abortions were allowed
http://voices.yahoo.com/history-abortion-346991.html
and
http://americancreation.blogspot.com/2012/04/founding-fathers-and-abortion-in.html