Yes a dog can mate when not in season, it's a rare event but it has happened. I have known 2 de-sexed dogs to engage in the act.
Also I have observed my Bull Arab bitch stand for my male husky. I found it interesting as she was the one to initiate the whole thing.
Her the body language was more subtle than a bitch in heat. The other thing was, she wouldn't tolerate any other male mounting her out of season, still there may be some reason behind this I don't know? He may have had a pheromone she really liked why it happened, I will never know.
Hope this answered your question
Just some general advice.
if you have a Female that seems to attract males out of the normal seasonal periods it's possible she has a uterus infection or Pyometra and should be taken to the vet immediately.
Should you be concerned about unwanted litters but don't want to de-sex your dog ask your vet about tubal ligation or for the boys K9 Vasectomy. I'd recommend doing lots of research before making decisions.