This is extremely natural, and the explanation is simple. Usually when we touch our dicks, before starting PE, it’s for masturbation, so when your touch your dicks in order to stretch, your subconscious figures out you’re going to masturbate, and gets your dick ready by going erect.
Just keep doing PE, and eventually, after 2-3 sessions, your brain will learn a new state of PE, in which the penis has to be flaccid even though you touch it, and will keep your penis flaccid, unless you think some really filthy thoughts, or have a girl strip in front of you… In which case, why are you bothering with PE? Just give her what she wants ;-)
PE isn't a chore. PE is a lifestyle.
If you feel down sometimes, come live inside my mind. I promise you'll enjoy the experience.