The part of this story that struck me is that it would appear from what the OP wrote, that the doctor walked in and right off the bat started talking about the weight reduction. It's almost as if he'd read her chart and made a decision before opening the door. My weight over the years has gone from very thin to PDFat. (pretty darn fat). I mention this as I've noticed how my size affects how others talk with me (or rather, talk AT me..). Sometimes I've thought that if somehow, during a fat phase, if I managed to drag myself into the office with my servered head tucked under my arm, the last words I heard would be the medical people saying " You wouldn't have this sort of problem if you kept the weight off....."
But the OP's question was "Was this doctor rude and if so, how to handle it?" That's a tricky question; technically, he was within his range of duties to bring up a possible health issue and a possible solution and as long as he spoke politely, I'd vote it wasn't exactly rude.
But....... did he make you uncomfortable by bringing it up before he'd examined you and made more of an obvious effort to learn your medical history? Yes I'd say he did that and also agree with other people that this smacks of him standing to benefit his referrals.
So it boils down to how you wish to handle your partnership with this health care professional. Personally, if I was in your shoes (and actually I HAVE been there) I'd write the doctor.
I'd tell him that how he handled this made you very uncomfortable to the point that you're choosing another OB/GYN. Stress that it's not the 'need to lose weight for health reasons' advice you objected to but that he appears to have made the decision to make an aggressive referral for major and EXPENSIVE to a doctor with whom he appears to have a connection before any exam or medical history discussion. I'd also add that he gave the impression he was more interested in getting you to this doctor than your OB/GYN exam and you couldn't help but wonder if there was a financial incentive in it for him.
Pick a new doctor and send a copy of your letter to the insurance carrier. IF he is getting a kickback on these referrals, it's the insurance company who'll be footing the bill for the gastric bypass procedures PLUS the complications that can happen.
When I say, I've been in your shoes, I really mean it! I had a staff person do the same thing. When she 1st began, I tried to cut her off with "I've already checked on it and I'm nearly 40 pounds under the minimum overweight amount that my insurance company has set to cover the surgery. She suggested I gain the 40 pounds and when I checked, I found it was her brother in law's practice getting the referral. I was furious and waited a few days before writing.
Do make sure to get on blood pressure meds if you need to while you find a health care partner with whom you can work. PB