It is a bad idea to go thinking of somebody as being an interim partner until you can find someone you consider to be "better".
I was on the receiving end of this a few years ago, I went out with a girl for whom I was her first real boyfriend. She was a virgin when she met me and had been to an all girls school and didn't have that much experience of males. We had a nice relationship and I was good for her, and she told me after the relationship was over that she was grateful for the way I'd helped build her confidence and make her feel valued.
Midway through her first year at university she had a work placement as part of her course, and she ended up meeting a guy who was older than me, had a good job and a bit of money...and she dumped me ruthlessly to get with him.
I managed to live without her ok.
I suppose I respect the fact that when she met him she went crazy for him and couldn't control that - but maybe I felt I was owed some loyalty. She obviously used me as a confidence builder when she was single and jettisoned me when she thought she had a better offer.
They stayed together for a year and a bit then he got a better offer (of employment, in a different part of the country), upped sticks and left her.