It's very random, but I was watching the Christmas episode from last year when I noticed the Doctor had a lipstick mark on his face just before he `married` Marilyn Monroe, hence this idea came into my head! So here it is! :)
With a fresh lipstick stain on his face, the Doctor ran over to Kazran and Abigail. "Guys, we've really got to go quite quickly. I just accidentally got engaged to Marilyn Monroe. How do you keep going like that? Do you breathe out your ears? Hello? Sorry, hello? Guys, she's phoned a chapel, there's a car outside, this is happening now!" The Doctor ranted circling the couple who were currently lip locked.
"Yoo-hoo." Someone called, it was a female voice no doubt and it was the person the Doctor had `accidently` got engaged too, but it wasn't Marilyn Monroe.
"Right, fine, thank you. I'll just go and get married then, shall I? See how you like that!" The Doctor says to the couple. "Marilyn, get your coat!" He continues now talking to the unknown female, that certainly wasn't Marilyn.
What few people would realise is that the lipstick mark on the Doctor face was no normal lipstick mark. It was halucogenic. Hence by the laws of America, after getting married in a small chapel, the Doctor and River Song were officially married. Even if he did think she was Marilyn Monroe.