Advice wanted / Can men and females truly be friends?

In other words, can men and females be friends and not have any attraction? I used to adamantly say, "Yes, of course!" But, now I'm not so sure.

I don't have many friends I actively talk to on a daily basis because I'm an introvert (i.e. I get drained by constantly talking with others) and am okay with spending time by myself. So, whenever a person (male friend) texts me frequently and instantly, a few of my flags raise. Recently, a male friend has been frequently texting me and I am quite comfortable with him, but I do not have any romantic attraction to him at all. He often compliments me and wants to spend time with me, but I don't want to lead him on. And so, I explicitly tell him that I only see him as a friend and that I do not want to lead him on. He says okay, and then keeps the same behavior...

I don't know if this (saying I'm cute, that he misses me, etc.) is normal for friends, or not? We still talk on a daily basis, but I (my gut) still feel as if I am leading him on although I already made it clear? Sometimes he feels too clingy and I don't know what to do. I want space, but I don't want to be mean. In the past, I went through the same scenario and felt the same way (the gut feeling and clinginess) with another male friend, who did confess he did like me although I did not.

Why does this happen? Do I have a lack of boundaries? Has anyone been in a similar situation?

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Thank you for listening (er, reading)!

(Disclaimer: Or people of different orientations. I'm sorry if I come off as not being inclusive, I don't mean to offend!)