Not if you look at it like I do.
They are all deviant sexual practices, so that comparison is valid.
Honestly, if you look at it, the Left has absolutely no reason to prohibit incest, so long as the participating members are capable of consent, except that they might also prohibit it where there are unequal power relationships in play (such as parent-child incest).
The traditional prohibition against incest is likely to have been to reduce the likelihood of reinforcing negative-recessive genes, however given the Left's die-hard support for birth control and abortion, such a prohibition cannot stand. And given that their ethical system is explicitly Utilitarian, lacking moral absolutes altogether, they have no reason whatsoever to keep a 17-year-old boy from having consensual sex with his 18-year-old sister.