So, I went back to re-read it and I didn't quite get the impression that she was saying "A relationship where you haven't done anything but kiss is noble, unlike those relationships where you DO have sex." This could be me forcing my own interpretation onto it, but the way I read it was more like "It is noble to still be in this relationship, DESPITE only having kissed." Basically, understanding that humans have urges, and for them to still be in love, despite having not done anything more than kissing is noble. This is, of course, disregarding the idea that they might simply be an asexual couple, but asexual representation is already so sparse and hard to come by, even members of the LGBT community tend to be pretty ignorant about it.
I feel like the girl's comments about her non-sexual relationship being "pure" were supposed to be seen as funny. Like, perhaps SHE was being serious, but the person that's supposed to be author, to me anyway, appears to be making a face that suggests "Wait, are you serious?" Her hypocrisy is supposed to be humorous, not taken as some kind of gospel.
To me, this comic isn't making a statement on the purity of sex, even comedically. For most couples, sex is an important part of the relationship, enough so that it can make or break it. For them to have an attitude of "we don't really need it" is probably what she's interested in. The idea that the love extends beyond sexual urges, which is what the author feels is noble.
That isn't to say I necessarily agree with this message, either, in that I feel like sex can have varying degrees of importance depending on the individuals in question. But I don't feel like it's trying to say that couples who choose to have sex are impure, unclean, or whatever else.