I'd be curious to hear if Thompson has an explanation. There might be valid reasons to object to the bill. As far as I can see, it commits the federal government to recognizing as legitimate a marriage between a 40-year-old man and his 8-year-old bride, if it took place in Yemen with her parents' consent.
That is not a reason to "object to the bill". That is a reason to "offer a common sense amendment to the bill" as pertains to a lower-age bound of such codification.
As such, while it is a reason to offer an amendment it is merely an
excuse to object to the bill entire.
For all we know a committee Republican offered a lower-age bound amendment, a committee Democrat said it was racist, and then everybody treated the issue as toxic. That isn't the point. Based on generic ecological considerations, if a bill has one problem a non-lawyer can see it's likely to have ten a lawyer would spot.
A more plausible non-hypocritical explanation would be that he has no problem with gay marriage for Pennsylvania but he's a "states' rights" extremist and doesn't think the feds should be imposing a nationwide standard. As thebeave points out, the default assumption for a politician is that he's a hypocrite. That applies regardless of the topic and regardless of whether he looks like one. Just saying we ought to hear a guy out before we convict him.
However, this bill also allegedly requires us recognizing the marriage of 40 year old men and 8 year old children from Yemen. Which is pretty darn common. More common than gay marriage. I know, I was like... 'man it is that common?!'
It's not obvious that bills should be judged by considering only on their effects on the most common cases. As for how common it is, I couldn't say; but for every example the Western media reports there are probably at least ten we never hear about because the eight-year-old girl didn't die of pregnancy complications.