Why is being gay considered more sinful than being divorced or commiting adultery? I am looking for Biblical evidence that homosexuality is a greater sin than adultery or divorce (for any reason other than adultery).

Prohibitions of "sexual immorality" could cover homosexuality, and I understand that Christian marriage is to be the joining of a man and woman as long as they live. I have also read Paul's warnings about men acting/dressing like women and vice versa, but these still don't seem to be outweighed by the passages relating to adultery. 

Many Christians are opposed secular gay marriage but do not seem to oppose secular divorce that can set aside a Christian marriage for "no fault".
It is worth noting that while divorce was not part of God's original intent / design, for those who were determined to divorce, God made a provision (through Moses) for them to do that in a respectful way.
This is in stark contrast with homosexuality, which, throughout the entire Christian canon is characteristic of deep depravity (cf. Sodom, Rom. 1), and leads to the severest condemnation and judgment from God -- every time it is mentioned.
Sin is sin, but one is tolerated (cf. polygamy), and one is severely, consistently condemned (cf. idolatry.)

Regarding the alleged distinction between orientation and practice, we should probably consider Jesus' Sermon on the Mount, where He explicitly refutes such thinking and defines even perverse thoughts as sin.
So what is a &quot;homosexually-oriented&quot; person to do?

those who belong to Christ Jesus have crucified the flesh with its passions and desires. -Galatians 5:24
we are taking every thought captive to the obedience of Christ -2 Corinthians 10:5
Set your mind on the things above, not on the things that are on earth. -Colossians 3:2
Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things. -Philippians 4:8

...which, incidentally, is good advice in general for any such struggle with the flesh.