I started my training later in life, at 41 years of age. I'd been married for seven years by then.
My wife doesn't train, though she may take it up one day.
Quite frankly I'm glad I'm not a single person training and looking for a mate. My experience has been, at two different dojos, that folks tend to spend alot of time together on and off the mat.
I think it would be difficult to seperate one's feelings if a romance blossomed and then withered in that environment.
But hey...that's me. It took me until I was 34 years old to figure out how to be a good partner in a relationship. And in reality I'm still working on it. Good thing I married an Irish Catholic girl; she won't dump me and that forces me to "train" to a new level all the time.
I have seen great romantic relationships develop at dojos. Certainly there are many partners who train together. I guess like everything else, it's pretty much an individual journey.