I'm just curious to see how everyone here made it official with their girl.

Did you say "be my gf."
Did you say "will you be my gf?"

Was it during sex or on a date or was it just while you were shooting the shit on the couch?

What was the conversation like? Did you lead up to it or just pull it out of thin air?

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AFC Level 1

I usually say "Now you're my girlfriend" once I have some comfort and rapport built. It doesn't really mean anything. why the fuck would you want to make anything official? lol just go with it and have as many girls as you can until you find THE BEST (there is no such thing as "the best" )

I surprised her with a picnic when we were on a run. I had a picnic basket, a blanket, and some good foods. Then I played her a cute song on ukulele and asked her to be my girlfriend in a playful way. Just avoid being overly formal, that shit ain't cool.

I never said to any of my girlfriends anything like that. I just do my thing like we were dating. They will bring this up. Girls always do.
My last one simply asked: "Are we official now?" I said, "ye sure". That's it.
The only one I've tried to get to be my girlfriend (was friends with benefits) got cold when I brought up the topic and dumped me quickly.
Don't ever ask the question "Will you be my girlfriend ?" That's quite bad. If you really want to take the initiative say: "You are my girlfriend". Lead her, don't ask.

Agree with AFC.

If she doesn't bring it up, you are probably doing something wrong.

Also, actions are much more powerful than words. So if you are already hooking up or have hooked up, then its normal for you to, say, invite her out to a dinner where she is going to be your "date." This implies that you want to introduce her to your friends/family which is a huge deal for a woman trying to tame you.

I'd say most of the time the girl will wonder "what are we?" earlier on than the man does. Timing is everything. Just like a kiss: if she is looking up at you, go for it. If she keeps looking away or avoiding it, don't kiss her. Similar logic here.

I said nothing until she asked "is it ok if you only kiss me and no one else?" She later texted "does this mean we're bf/gf? I dunno what the social conventions are anymore lol"

The feeling had been strong and mutual leading up to it. The girl will likely bring it up (so as to not feel used or betrayed by polyamory).