If you are just being friends with someone so you can get them to date you that is an asshole move. Because if it turns out they don't want a romantic relationship with you more than likely the "friendship" will end. Leaving both parties feeling cheated.
If you were friends but your feelings grew to romantic that's different just remember if rejected don't toss your friendship away. Remember how you were friends before you felt more for them.
I don't think friendship is the right thing to compare to sex. There's a lot of power in sex, yes, but the exchange of that power is more widely distributed, such that in any given relationship, any given exchange, the transaction of sexual availability may be with friendship, or maybe with financial availability, or reproductive potential, or daddy issues or whatever - friendship is only one possible currency for sex.
All interpersonal relationships, healthy or not- even in codependency or other issues- are mutual exchanges. One person enhances another’s life or fills a practical/emotional/psychological need for the other. We do not engage in relationships with a person of any sort unless we are Recieving something or feel we are. The fact they are exchange based doesn’t inherently mean they are transactional however.
The language of the original post shows a clear lack of understanding to interpersonal dynamics. When we say: “using them for sex...” there is an implication of one sided or abusive conduct. For instance- misleading a person into thinking a relationship is based on more than sex when sex is your only interest. Now- a key differentiator with friendship is wether you are “using a person FOR their friendship...” or using a person by leveraging a friendship that only exists for altering gain.
The irony there, is that the guy who is bitter at being “friend zoned” is using the woman’s friendship as a tool to try to gain leverage for sex/romance. It is not the woman who is using him, unless she has made promises or allusions to being more than friends without sincerity.
When we “use a person for their friendship” the meaning there is simply that we are concerned only with what we get from the friendship and not with reciprocating the friendship. Not having sex with a person is not using them for friendship- it’s being a friend who doesn’t have sex with their friend. Being a good friend but not wanting to be more than friends or sexual friends is not “using a person for friendship” it is simply... being a friend.
Tl:Dr- the man who uses friendship as a means to try to leverage sex or romantic relationships is using friendship. The woman who wants to be friends and is a genuine friend is just being a friend. Sex is not a promise of friendship or even implied. Using a person as a friend is simply taking from a friendship without caring about wether you yourself are being a good friend.
If you were friends but your feelings grew to romantic that's different just remember if rejected don't toss your friendship away. Remember how you were friends before you felt more for them.