It's probably referring to somebody else (rather than the speaker themselves). It's not impossible that it's referring to the speaker themselves, but unlikely.
Anyways:
本当に頭を使わない男だな
You really don't use your brain (or he really doesn't use his brain)
Now to:
もうチンコだけになってしまえばいいのに
〜ばいいのに translates roughly to I wish 〜 happens. For example, you can say 帰ればいいのに、もうずっと住めばいいのに and so on. It's often used to convey feelings/yearnings (rather than literally describing ones actual wishes). For example: 明日から戦争のない世界になればいいのに、みんな億万長者になればいいのに
もう translates roughly to "given the situation". When combined as もう〜ばいいのに, it roughly means "Given the situation, they should give up and 〜" or "Given the situation, they might as well as 〜". It's hard to explain but basically implies something has build-up which would justify 〜.
For example, if you say 退学になればいいのに it simply means you wish they get expelled. If you say もう退学になればいいのに you imply there was some kind of build-up that justifies the expelling (for example, maybe they kept not showing up to class etc.).
Back to the original sentence, the speaker first points out the person really don't use their brain (and thinks with their penis) habitually ("You are those types who really don't use their brain!). Then they imply if you hardly ever use your brain and instead listen to your penis all the time, you might as well give everything else up and become a penis.
