Of course they should. Don't be ridiculous.
What age would you choose (and why) for this activity to be legal?
18. Unless both are minors.
What about other "crimes against nature", such as bestiality and incest?
No and no.
If you support keeping them illegal, while you support legalizing fornication, adultery and homosex, why?
I would prohibit bestiality as a form of animal cruelty. Bestial relations are almost always rape. I would prohibit incest due to the inherent sociolegal asymmetry of the relationship and due to the dubious consensuality in most cases.
How about polygamy and gay weddings?
No and yes (but not via judicial edict or by ignoring the laws).