“Let’s have fun” would probably mean to me “he’s not very serious” about a relationship. (It would depend on the rest of your profile. I would take everything into consideration.)
I must add: I think “let’s have fun” is a totally acceptable thing to write. I’m probably the unusual one who would interpret that phrase as meaning un-serious about looking for a relationship.
Heck, “friends first” is a death knell for me. I’m out! It confuses me too much!
I believe it can mean anything from: we’re going to be friends for an indefinite amount of time. Heck, just now I tried to flesh this out more and I stopped myself. Because I find “friends first” TOO CONFUSING! My brain just can’t compute being on a dating website but writing “friends first.” For many folks that approach seems totally natural. To my INTJ-self it is the epitome of confusing.
I feel like I was very unhelpful in responding to you today, Jim Fritzen — my apologies!
Many thanks for your kind congratulations and being an ongoing follower of my stories!