My boyfriends strange measurement obsession.

warning: contains detailed sexual discussion

K I had some questions about my boyfriends strange sexual turn on and want to see if anyone has any proper answers what the deal is here as I don't know what to make of it. I hope its more common than I thought. I hope to hear hopefully from someone who knows perhaps about the psychological understanding of this strange sexual turn on for a man. Sorry if its a long explanation but I need to be thorough with what is going on.

Alright so me and my boyfriend have been together for around 8 months and are both sexually active and have been for the last 4 months. We are both 18 and love each other a lot.

He has always puzzled me with a strange fixation of measuring his penis as a way for him being turned on . I will explain further. Often before getting into proper sex we will be in the preliminary stages of undressing each other whilst kissing and touching and then he will often lay on his back on the bed when he is....ummm..."ready" if you know what I mean. Then he gives me a ruler and asks me to measure his erection which usually is not fully erect straight away but is usually 6 and 3 quarter inches fully erect. Then he seems to want me to taunt or to tease him over his erect penis size as it grows. For example he will want me to measure him while he is not fully erect and run commentary while his penis becomes harder and longer so if his penis is coming up to 5 and a half inches hard he will want me to look right into his eyes and taunt him over how it is coming close to six inches and raise my eyebrows whilst taunting him with stuff like "we almost got sixxxxxx....". Then when past that he will want me to taunt him further on how it is so close to seven inches but yet not quite there, he also wants me to encourage him somehow like by saying stuff like "come on give me seven proud inches baby..." as I am caressing his balls or running my hands up and down his penis. Some may see it as a way of him trying to issue a challenge to see how much I can please him to get the most "turn on" out of him but it its not that. It does seem to me to be solely about the size and talking of how hard it is getting and what mark it is coming up to on the ruler, that gets him highly aroused. I don't often even have to touch his penis and he will just keep getting crazy as I commentary his measurements as he looks into my face. Its all strange. After a while we will go back to straight normal sex but he will always insists on doing this at least once every session. I don't think he is trying to brag either as his size as we both know is in the fairly average range.

I did ask him why he wants me to do that every time and his honest reply was "I don't know but there's something amazing about being taunted like that with my manhood that just gets me so wild".

Its important to note this did not happen when we first started getting intimate as I am sure he knew if he did ask me to do this too early then it would be weird lol. No it started around 2 months after frequent sex. I consider it a kind of a roleplay but its just strange lol! but I do it just so he can be turned on so we can carry on the action. I don't mind to much.
I just want to know why a guy would be so turned on by something like this which is so weird. I am looking more for the deep psychoanalyze type response about men but anything is appreciated.

Thx.