How to tell my family that i'm bi

It sounds like your family was pretty accepting of the idea of you being lesbian. It would seem like they are semi accepting of it. You could explain it to them just as you've explained it to us.

Keep it light: how about "Guess what, I just fell for a guy".

If you're not concerned about it they probably won't be, given they didn't freak about you being with a woman.

I think you should just tell them you're bi before introducing him to them, then afterwards introduce him.

Honestly if they were fine with you being with a woman, they should be more than okay with you being with a guy. Being lesbian is more weird and surprising to them so if they accepted that I think your fine. You've got nothing to worry about.

Make sure you know what YOU want, before you confuse your family, any further.

Tell her “baby it ain’t no lie, I’m bi, bi, bi.”

Well, I have never really came right out to any family members myself and held a sign " hello family, btw - I'm a crossdresser". Except I did tell my sister early on. The rest I felt they were to judgmental to even worry about telling them. So in your situation, you can always just say that it's a woman's prerogative to change her mind whenever she feels the need. So you can always tell your family you haven't made up your mind yet, but you are working on it. ? I just know my family member is a pastor and he could not really even talk to someone that was maybe gay, bi or lesbian. well they had a good one, her daughter grew up to be lesbian. They had hard time accepting but they did.
My point being they are your family, after the dust settles and they try to figure it out, chances are they will still love you no matter what. I'm betting they will come around after a bit. Just let them ponder it for a while. It's just an adjustment period for them.
It took me 25 years to finally be able to accept myself. But I couldn't expect others to hear it and the next minute accept it. Give them time, and you might want to tell them ahead of your date with Mr. Right, or is it Mr. Right Now ? Just kidding. Best of luck.

Why not just tell him straight out yourself. Be honest, he might respect you more for your honesty. Besides it might get you both talking about some of your fantasies. Just be honest, tell him yourself and go from there.