That's a good question, and I don't have much time to answer right now, but I'll say what I can.
What exactly do you mean by legal bonds and casual relationships? If by legal bonds you mean civil marriage but not a religious service, that too would be unacceptable by the Church. Casual sex is condemned by the Church whether it's homosexual or not.
LGBT people can absolutely love and be loved, but love doesn't always have to consist of sex. People can certainly be friends with people of the same sex and love each other, but they just shouldn't have sex. Also, homosexual feelings are not sinful. Only acting on them (i.e. having sex) is. People homosexual attraction are called to live chaste lives.
As to why, I'll give three reasons:
If you're not Christian...
There are ways that we can arrive at the conclusion that homosexual acts are not moral. Homosexual unions are not natural because they can never result in a child, and one of the most important parts of a marital union is procreation.
If you're Christian...
There are various passages in the Bible that condemn it:
Romans 1:26-27
1 Corinthians 6:9-11
1 Timothy 1:8-11
If you're Catholic...
The catechism condemns homosexual acts in CCC 2357. To get a better explanation of why, I suggest reading CCC 2357-2359. It's less than a page and it's explains why this is what we believe as Catholics.
I hope this was helpful, and I'll happily answer any other questions you have. God bless!