In terms of legality, I think the legal line should be wherever there is a clearly definable societal interest in limiting a behavior. If there is not articulable societal interest, then a behavior should not be infringed.
So you are comfortable with incest and interspecies sex too? How do you define societal interest? Since male homosexuals have an average life-span of about 45 years (on a par with IV drug users) and the cost to treat the medical conditions associated with the lifestyle exceeds normal lifestyles, should we draw the line or pay the costs? I for one, don't care to be forced to pay the cost for a destructive public health issue that is a personal choice. Furthermore, AIDS patients are dominated by male homosexuals and are breeding grounds for highly resistant bacterial strains. In short, male homos especially, are a huge public health issue.