For the huge demand among men - it's best to think about an AI girlfriend as an alternative to going to the strip club.
Sure, some guys go to strip clubs for just staring at naked women. But a huge (mostly unacknowledged) part of the appeal is the short-term companionship mixed in. The flirting, the ego stroking, all of it, just an attractive woman taking an interest in you and talking with you. Lots of guys I have talked to have had the experience of one of their buddies getting too drunk and "spilling their guts" to a stripper. That's not a thing that happens if you're just there to stare at bewbs.
In Asia, hostess clubs operate much more along the lines of this idea of emotional fulfillment. Strip clubs are part of the overall entertainment service sector, but really focus more on the bewbs aspect. In the West it's much more mixed in together instead of in two separate types of businesses.
It's a lot easier to implement an AI version of a strip club or hostess club than an AI version of Twilight or 50SoG.
I wouldn't be surprised if the overall market size (in users, not dollars) for all types of "relationship substitutes" for men is simply far, far larger than the market for women, even though Twilight and 50 Shades are themselves huge. For every 1 female erotica fan out there, I bet there are 10 or maybe 100 male users of porn or OnlyFans. I would also guess that the amount of money spent by men is at least a multiple of the amount spent by women. If I was an entrepreneur going into a market where, say, there were 100x potential male users and maybe 3x potential male dollars, I'd be a fool not to at least initially target men.
Also, even if we were to call it a "relationship", I'd say it's a lot easier to cater to the typical male wants of a "relationship" through the current (and near future) state of AI than the typical female wants.
Disclaimer: The usual stuff about "not all men/women etc. etc."