Next time something like this happens, tell your friend, "Hey, lets talk about this later. I am going to buy you a beer tomorrow. We can get all the details then." You should inform your friends that this means the topic is going to offend someone and "getting a beer" should not be your "topic not girlfriend friendly."

You kind of fucked up. 20lbs, depending on how tall the girl is, doesn't seem like a lot and can emotionally hurt females when you phrase it like that. I would be offended if a guy I was dating told me, "well, 20 pounds makes you unlovable." Because people gain weight due to health problems. Plus, if your girlfriend is the baby making sort, she might have freaked out you would think she was ugly if she got pregnant. There are a lot of reasons your girlfriend acted the way she did.

She might not have handled it in the best way, but now you know her limits and you should keep comments like that to yourself or private. Your friends and you sounded like jerks talking like that because it makes you look shallow. Trashing someone for weight gain might be okay in the man cave, but a lot of people see it as crass.

I completely agree it is your friend's right to dump someone who doesn't follow a standard of living your friend feels is important. It is a completely other thing to imply the worth of the girl decreased as the scale increased. It might be true, but its not something you talk about in polite company.

So I would apologize to your girlfriend and explain you were trying to sound supportive, not shallow. Tell her you would like her to have some sort of phrase she can use when she is starting to feel uncomfortable. "Could you get me a drink" might be one of them.