I guess it could be offensive, but keep in mind, she doesn't have kids yet. Many people's stance on being a parent completely changes the second they actually hold their first child. She's single and childless...she just doesn't get it yet. I don't think she understands the bond. And considering everything she's been through trying to lose weight and maintain, I can understand her being concerned.
I was morbidly obese before having weight loss surgery. I lost a lot of weight and got healthy. One of the reasons I did it was because I wanted to have a baby and that just wasn't possible with all the extra weight I had. Even though I went into it with the plan of wanting a child, it was quite the mental transition to prepare my brain for putting on any weight again (even though it was healthy & related to a much-wanted baby.) Yes, I got over it and it was more than worth it. But I can't fault Jillian for being scared about putting herself into a situation where she would put on weight. Maybe she'll get over it. Maybe she won't. If she's happy with adoption, then that's great and what works for her.
That said...it would have been in her best interests to keep her mouth shut about that stuff. Like other posters have said, she could have made a mint of pregnancy workouts, get your body back DVDs, etc.