I mean, to be that guy, it sorta depends how you interpret the definition of platonic and perhaps further out interpret the definition of friendship.
If you're defining platonic as a pervasive feeling, then wouldn't too the second you felt someone was attractive even if it passed have ruined the potential platonic relationship? And further, what if one were engaged in a particularly platonic relationship for years, but suddenly felt a brief draw due to their friend being particularly attractive that day, ruin forever a platonic relationship? That just seems like a very strange way of defining it that seems to purposefully fail to account both for how humans are.
I see platonic relationships as much more of an active choice between two people to pursue a relationship in which their sexual desire (which may or may not exist at all) is not primary or even on the list of reasons they retain connection. Thus my claim that friends who do have that attraction who do not act on it are clearly valuing a platonic relationship far more than people who never felt that urge in the first place.
The question of what is mutual is also an obvious muddy area. What if a boy is absolutely head over heels in fancy with me and I see him as simply a dear friend? (This has happened more than I care to recount. I may need a sign if this keeps up.) Does his lack of platonic feelings make it impossible for our relationship to be platonic? I don't actually know the answer, but I do think that what each individual intends is probably important. I can say for a fact that in one case, he confessed his feelings for me, I gently made clear I did not return this feeling but badly desired to remain friends, and he agreed our friendship was too special to ruin over that. We went on to maintain one of the best friendships I've ever had for nearly four years. I do not think the platonic relationship goes out the door the moment attraction comes into play. I just don't see things as that black and white or all or nothing.
As for why defining friendship is relevant, well, perhaps this will explain it. I myself, tend to go with the Aristotelian model that our friends are essentially aspects we take onto our own identity and that thus for one's friend to be harmed is for one's self to be harmed and all else that idea implies. Without defining what is attractive to me (that's a whole new can of worms), I think the following might explain why this is significant.
I hope I'm not being arrogant to say that I find myself quite attractive. I could list all of the reasons why, but like Rose in "Eight Cousins" it is sufficient to say that I do not think I'm horrible to behold, and even find my appearance quite pleasant to see. I find all that to be doubly or even more true about my personality and intellect. I would fall in love with me and want to um, ya know, the other thing me.
The friends I take on are, as I have said, new aspects of me. And while I may not find each physically attractive exactly or they aren't all lights out hotties, I would say that in each case, there is something or many things desperately attractive and romantic about each of them. There would have to be, otherwise why in the world would I take them on and love and protect them as I would myself?
Thus, I would argue in the cases where a strong friendship has developed, it would be nearly criminal of me not to recognize that which is attractive, sexually attractive and romantically significant about the people who are my friends and be attracted to it. It would be tantamount to an external self-esteem issue. It would be like looking in the mirror and thinking "no one will ever want that." Instead, I would argue it is proper to recognize and value such things, without letting them drive or particularly matter within the friendship.
