Not a homophobe - his point misunderstood
Watch this interview. Irons supports equal rights to all couples and says so clearly.
His point was that he feels lawyers will have a field day with gay marriage due to issues such as incestuous relationships.
The reason why gay incestuous relationships differ from straight ones is that they are not accompanied by the potential of procreation. Irons believes that the only correct argument against incest is that applied to matters of procreation, and appears to doubt the validity of other moral arguments against incest.
With gay marriage, incestuous relationships between two men will be more difficult to disallow because without the procreation factor all that is left are other moral arguments, most of which are bound to be rooted in religion rather than empirical science.
Irons believes that this will lead to some very complicated arguments, but I couldn't even tell from his words whether he is against all forms of incest or not. It appears that he sees an issue with incestuous marriage due to the inevitable legacy tax avoidance, a valid point that most people probably did not understand. He did not equate incest to homosexuality either and generally sounded very supportive of gay relationships.
In general, like Jeremy, I see laws against incest as inherently problematic and subject to inevitable debate and change. Incest carries a powerful yuck factor, but outside of the procreation matter (genetic implications) it is difficult to muster effective secular arguments against the union of two consenting adults involved in an incestuous relationship.