When does a crush become more than just a crush?

Anonymous
So my friend told me he has had a crush on me for a really long time, which is cool because I have had a crush on him too. After being in a really long relationship, I am finally single so he decided to tell me. He has a girlfriend and they're in an open relationship. We have been talking a lot more, especially about what we're going to do about this. I suggested no-strings sex, except now I'm wondering whether that would be ok. He's had this crush for a really, really long time and I guess I'm just worried that it's more than just a crush at this point because we've been friends for a long time and have gotten to know each other very well. I don't want it to become more and when I told him that, he said things like, he really likes that we're friends, and that he doesn't know where it's going to lead, but that he doesn't want to hurt me either.

So when does a crush become more than just a crush? What are some signs that it's more?
Updates:
+1 y
Also, he lives with the girlfriend. They've been together for about 9 months now. Began living together after about 6 months. He talked to me about that because I had lived with my ex too.
+1 y
I told him that expectations rise once you start living together and he told me that they had talked about it and both knew it was more of an arrangement than anything, so expectations for seriousness wouldn't rise, but I don't know.
+1 y
Can you live with someone you're not that serious about? I thought it was a very quick jump into it, but I just don't know how to feel about it all.
+1 y
Let me clarify, they decided to move in together over the summer, like May/June. Then in August after my breakup happened, he told me he wasn't sure about it. I told him to do it as long as they were both clear on the expectations they had of each other.
+1 y
He pretty much told me it was just for convenience and that they both knew that. Then a month or so after the breakup, he started contacting me randomly until he told me about his crush and then told me that they were in an open relationship.
When does a crush become more than just a crush?
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