Therefore, to answer: what you've received is most likely spam.
A quick search for found that they've used several Florida-based Google Voice phone numbers to spam random cell numbers with the message, "Hi I put a pic on my page on just sign in and check it out ;-)".
I'm answering the question with this assumption in mind, although, I'll cover the other possibility at the end of this post.
Could this dating spam have come from your boyfriend's former adult dating site use?
It's possible, yet more likely that your cell number got thrown into some database somewhere (similar to emailed internet dating scams).
You may even find that online dating isn’t right for you – if that’s the case, all the better to find out without investing much into the process.
Free sites generally offer more flexibility for different types of relationships, including casual hookups and same-sex relationships.