When I am in love with someone, I'd rather talk to them intensely for 1 hour, or cuddle or look into each other's eyes, than have sex. You want the complete romance. Saying someone is just out to have sex when they want a honest true romance is demeaning.

Yes, they may want more and they may want to steer a platonic relationship into a romantic one. But why is that blamed on shallow horniness? If the other person wasn't an interesting person (whatever they are attracted to personality-wise), it also wouldn't be happening.

As for the OP, normally I would say, don't blame yourself. If you have an attractive personality and are good looking, a lot of males are going to notice. And they are going to try. They can't help themselves. If they are attracted to you, it will hurt. And if you say that you are unavailable, but then flirt anyway, they either will be confused, or they won't be able to help themselves. Even if they would never be interested in you if you would cheat on your partner.

That said, the fact that this happens online, and in a sense is artificial, rather than offline and more natural, is maybe a bit questionable. If I was looking for a platonic friendship with a female, I wouldn't do it online. What is the context of this online medium where these friendships develop? It is a bit strange, I must admit.

Still, a relationship between a male and female is always going to be hard. Let's assume the two people are really soulmates. Why are they not romantic life partners? One of them, or both, just isn't romantically (read: sexually) attracted to the other. That is not a nice thing to have to admit to in a relationship between soul mates.

Even if there is a mutual understanding, somehow, that no romance is to happen, when you are single, feeling down, rejected even, it is nice to realize that your friend doesn't just see in you a friend, but also an attractive potential mate. I mean, if I am really close with a female, and I never acknowledge her femininity, her attractiveness, she should feel some insecurity about it, justified or not. And if she knows I find her attractive, and we are best platonic friends, why isn't something more happening?

And even beyond the feelings of the two people involved, what if one has a partner and the other is single? Do they get jealous?

-There are a few cases where there are no problems. The male and female friends both have partners, so everything is settled and the friendship evolved into a 4-way friendship.
-Both are homosexual.
-Both consider the other physically unattractive.
(-Both consider the other attractive in general, but unattractive to their personal taste.)

Personally, I want more platonic friendships with female. But I don't like the idea of having one with a not so attractive female that finds me unattractive as well. And I am single and not homosexual. That said, I have met females that I know are attractive, but who have left me cold. I am not sure if that is just because of their personalities. In some cases, their personalities were perfectly fine, as far as I could tell. I couldn't imagine something romantic happening between me and them, because of how they looked or because of their body language, even though they were nice persons and I could easily be friends with them. But I think that is rare and the lack of physical attraction could probably be overcome.

I would also hate it if my best friend was female and thought I wasn't physically attractive.

That all being said, since it has been 4 times, since you are in a relationship, I agree with evaluating why female friendships are a problem. My experience is that females are much more mature, richer in the diversity of experiences they are looking for, balanced, etc. They may on average be a bit less interesting, or intense, or maybe some other things. But for a platonic friend, ignoring romance, I think females have more to offer.