Sex isn't necessarily always "submissive," but a person can be submissive during sex. How they act and what they do will deem them submissive. Just because a person gets spanked doesn't necessarily mean that they're submissive (though in the way described, this individual probably is), they may just like having their tush tapped a bit.

I do think that there is a certain amount of vulnerability present in all sex, whether the person is dominant or not. Submission though, is entirely different.

I don't see sex as submissive at all. However, its likely you have heard that from a Christian DD site because most often people involved in those relationships are masochists. In DD erotica, the woman is usually portrayed as being a naughty child, whom is always wrong, and whose duty it is to allow her husband to use her. I have debated with a few CDD women who defended their right to be treated like dogs, insisting yes their husband really IS always right, and they consent to the treatment so it doesn't matter. While yes, they consent, I personally think some women have been pushed into these relationships as a simple "Hey sweetie, lets try something new" and then their inner "Well maybe I do deserve it" voice took over, since there are some things stated in CDD communities that simply are abuse in the eyes of anyone but CDD couples.

I'm not trying to speak for all of them- But I have to say you sure picked an extreme fetish to start exploring. CDD is odd to say the least- being that the public stance from most churches is that sex for pleasure is bad, and CDD, as I mentioned, encourages the woman being the man's plaything. In fact in some CDD tales I have heard sex mentioned as an accompaniment to punishment- rough, impersonal sex, where the woman is used as a hole and nothing more.

Once again not trying to represent all CDD out there. Just what I have seen.

As far as sex being submissive- I said no, but.. here's why. Well, I was raised to think sex was shameful, for a while I did have a masochistic mindset towards sex, But after some self discovery, and deciding that no.. I do not deserve meaningless punishment, and I especially do not deserve it from myself or from any 'partner', since if they were such a partner they would not attempt to do that to me. Partner implies an equal relationship, and in this way I view sex as equal. There is nothing wrong with BDSM if its consenting, and if it doesn't turn into an "I want out but I am afraid to say so" relationship- But I am not into BDSM in any form, and I value equality with my sexual companions, and heck, even my normal companions.

So I guess the easiest answer for me is that I think it's not submissive. I would never do, sexually, something I didn't feel comfortable with and my partners have enjoyed the fact that I will not force them to do things, either.

About spanking, it's a common fetish. Many girls enjoy it in some playful form or as roleplay in the bedroom. It's not submissive to want to be spanked... It's just a want, like everything else

Sex is rarely feels purely submissive for me. Hubby and I do play domination and submission games sometimes, but even when I'm playing the sub it's still what I want to do because he's taking the dominant role for me and letting me be submissive. Does that make sense?
Another thing is Tantric sex, which we've also tried. There are zero submissive elements in that.

No, it isn't in a sense submissive. It's sensual and erotic. Not necessarily submissive. A woman can be extremely dominant and enjoy being vaginally penetrated. A woman can also enjoy being dominant and penetrating their partner while being penetrated. I'm not sure why you made it seem like sex is only performed by heterosexual couples, but it's alright.

Sex may or may not be submissive. Personally, in any sex that I've had, there has been dominance and submission. But for others, there sometimes has never been that exchange.

I don't think that sex, for a woman in a heterosexual relationship, is inherently a submissive act. However, I think that women are sent a message from an early age that sex is something that is done TO them, not something they are active participants in. I think this idea takes away a person's sexual power. Granted, if it's a conscious decision and it's something that the individual wants, more power to them - but I see and hear a lot from women that make me think that for most, it's part of the bullshit they were fed as children, teenagers and as adults. If you're taught that your vagina is a sex toy that men want to use, then yeah, sex is submissive - you're a fleshlight with a face.
If it's an unexamined position, I don't think it's positive or empowered. If you're doing it on purpose to get your rocks off, then by all means, submit away. I prefer to be a full partner in mutual perversion.