It's good that he's been honest but the text message was not supportive in the slightest.
It's unlikely that you'll be a tiny size 6 again after two kids, it must be hard for you to hear that after all the time, the main reason he got with you was because you were extremely thin.
You don't have that sexual collateral anymore but as pp have said being thin and being beautiful/making an effort are not mutually exclusive.
However, it does not sound like your partner would find you attractive even if you middled out at a size 10 and you were taking care of yourself better. It sounds like he wants you at your rock bottom weigh, ultra thin.
Is he going to help you lose weight? Is he going to shop for healthy meals and cook them? Is he going to look after the children while you go to the gym 2-5 times a week? Is he going to be kind after if you have a slip up with your diet?
Yes, many posters are going to say that people are attracted to what they are attracted to and you shouldn't vilify him for that.
However, it seems that he is not willing to help me lose the weight losing weight or work on the relationship.
He just wants you to be a size 6 again but even that won't solve all your problems.
He does not seem to be gently encouraging you, you have had to drag this uncomfortable truth out of him.
Would he be happy living in a sexless marriage if you hadn't said anything?
What about you? What do you want? What size will you be happy at? Plenty of man would find you attractive the way you are.
You have had two kids that changes a woman's body, not necessarily staying bigger forever but even your bone structure has changed now. Even if you were a size 6, you might not look the same.
Im sorry for you that your partner does not seem understand this.