I've been on since 2011 and this has always been the case. And the site owner makes it able to to happen by not prohibiting them. If I recall, years back, the site owner gave singles the option to restrict teens and married (since the singles forum is 18 plus) and it was agreed to let them continue to come in.
The sites subforums seem to be more general guidelines than strict rules.
And since the Singles forum is not intended to be a place for people to hunt down spouses, but is more a general chat area, no one has a problem with them. And at times you'll see posters here offer married people to respond to posts to get their take.
So, no, it's not unusual at all here.