Go with an even easier case. What if it is of a 17 year old who is old enough to consent, even by the law where they live, who willingly takes the photograph and releases it. Technically illegal, but why?
Doesn't this holds true of the other crimes we were talking about taking place on the dark-net as well? Maybe not at the same rates, but even weapon sells don't definitely mean someone is coming to harm by that specific sell.
It promotes growth of the marketplace which also allows buying/selling of drugs that do involve victim crimes. If the market was limited to safely produced/trafficked drugs, it would be different. One might even make the argument that shutting down a market that only allowed victimless drugs end up causing harm.
Buying drugs does fund those actions. Drugs being illegal are a major reason why, and why I support legalization. But that doesn't excuse anyone currently making a purchase today.
Think of it this way, illegal images could be produced by advanced CGI, but aren't because that tends to be just as illegal. That doesn't excuse anyone who consume images not produced by advanced CGI.
They fund the ones supplying those drugs which involves a significant harm to society. Just look at people who are unwilling participants in the drug trade or those who receive funds from selling drugs to sell other things such as weapons or even humans.
(Yes, I realize legalizing local drug production and distribution would undercut the market, which is one reason why I am for legalization. But that does not excuse supporting the current cartels.)
Buying drugs can fund actions far worse than just creating drugs. From human slavery (drug mules) to murder. Some of the other things on that list are bad not because the actual item being traded is bad, but because of what the trade enables. I'm not seeing a significant difference between the markets in this regard, except for popularity.
Compared to the example you gave, where no one is harmed and no further harm is caused or funded by people being gay.
Edit to specify that this doesn't happen in every drug transaction.
I've seen a number of articles about college educated women having trouble finding enough college educated men to meet the demand. Also consider the factor age can play in this, given that people tend to gain wealth throughout most of their life. If women married men who were a bit older than them, you would see a marry up effect and you would also have more widows at older age since their older husband has passed.