And then there's the gay artist duo Gilbert and George:http://www.bbc.co.uk/devon/content/images/2007/02/27/gilbert_and_georg e_465x350.jpg
Although not moustached or bowler hatted, they have rather an air of the Thom[p]sons, in their deadpan manner, their formal dress sense and their inseparableness. (They're rarely seen apart and regard themselves as a single artist, apparently.)
I wonder if the Thom[p]sons, though probably not intended to be explicitly seen as gay by Hergé, have nonetheless become an influence on gay culture.
On the question of whether Hergé's Catholic upbringing would have made him antagonistic to the idea of homosexuality generally, he actually seems to have had a fairly relaxed attitude towards homosexuality, at least by the latter stage of his life. I'm basing this view on this passage from an article by comics expert Paul Gravett (reviewing a Hergé biography by Benoît Peeters):Peeters goes on to quote from Numa Sadoul's definitive book of interviews from 1975, in which he asked Hergé if the absence of feminine characters in Tintin might mask a repressed homosexuality. Hergé replied, without embarrassment it seems: "I don't think so. You never know. If I had tendencies towards homosexuality, I don't see why I'd conceal it."
Here's the link to the full article, which is interesting generally:http://www.paulgravett.com/articles/018_tintin_2/018_tintin_2.htm
Though not particularly concerned with the topic of this thread - the Thom[p]son's sexuality - some of the biographies that Paul Gravett reviews deal with the general lack of sexuality or family life of the Tintin characters and the theories for why this is so.
Of course wth the Tintin books being aimed (at least partly) at children, you wouldn't expect the sex lives of Thom[p]sons to be spelt out in explicit detail. But the idea that they can be seen as a long-standing gay couple who are so inseparable that they dress alike and even look virtually identical doesn't seem to me to be a completely invalid reading, nor one that is inappropriate for child readers to be allowed to consider. Many children know openly gay couples in their family or within their parents' circle of friends, so wouldn't be taken aback to encounter a gay or possibly gay couple in a work of children's fiction.
The theory that they are twin brothers who have decided to spell their surname differently is equally valid, of course. It may be a better theory, especially since they never use their Christian names (whatever they are) which might otherwise distinguish them. And it's a more realistic explanantion for them looking so nearly identical. But one of the great things about the Tintin characters is that we're told so little about their backgrounds that each reader is free to interpret them in their own way.