I would feel completely taken advantage of, embarrassed, angry, and “used” if they made it clear that was the ONLY reason they were with me. And they admitted not having any real feelings for me in that time…..They would deserve a fly kick to the face.
However, I think to a degree, when you are in a long-term relationship, you are using the other person in certain ways, whether you admit it or not. You’re using them for emotional security, for sex, for company, for simply that feeling that they elicit in you. If you weren’t getting anything out of this person, then you would probably choose to be alone, or be with someone else.
Relationships have to be give and take though. If my partner had been take, take, taking, with no giving. And I was feeling that, then I would break up with them.
However, thinking of situations I have been in, If a partner told me they were using me for two thirds of the relationship, I would probably reply “So have I…...”