Hello, Absoluteladoi, and
to GaySpeak.
I don't really know where any of this is going but you're both 18, you say, still teenagers even though you can now officially vote and are considered adults by society. I think your friend is probably very comfortable with you and being around you, which leads to some slightly 'improper' behaviour. Take it as a mark of friendship. You say that you are fairly comfortable with this penis size comparing and son on, and I guess you have seen one another's naked bodies before, maybe at football practise, showering? That may make him even a bit more comfortable with seeing you naked or semi-naked, and because lads are lads, and are used to occasional physical bantering, they are used to touching other lads' bodies and tackling them as if they were fighting. I don't think I'd like to be hit in the genitals if that's what he's aiming for, because they are sensitive parts, and a strike in the area could be painful. If you need to complain about it, tell him not to touch you in that area. At least, not in that way. If you want to be touched, it might be in a sexual or romantic way, how do you convey that to him?
What sort of insecurities are you seeing in your friend? It's not to do with penis size, surely? There must be more to it than that. This is the issue that I think you could potentially bring up with him. Tell him what you've observed and how you feel about it. It would be normal for him to be jealous of you if he had any reason to be. Does he have any reason to be jealous of you? Could it be anything about being more mature (in your head)? A bit more information on this matter might be useful. Note that penis size when flaccid has nothing to do with how a man grows when erect. There are those who are 'showers' and those who are 'growers', ultimately ending with very similar dimensions when in full erection. If you want to have an erection contest, then you've got to get erect, right?
To address the curiosity thing: well, you're both eighteen, with hormones raging, as they do at that age... and you're curious, I daresay he is too, but maybe not willing to admit it, because maybe it's a sensitive subject. At your age, if you haven't already done all the comparing stuff, and if you want to take it up a notch, then you're both 'adults' and you can deal with it in an adult way, ie not one that needs to be fraught with anxiety and guilt. I'd say you've already taken intimacy into a certain stage since you've both seen one another's penises, in joking situations, or dare situations. You might ask him why he feels the need to show you his penis doing windmills or other things. After all it's not something a person actually does in an 'all-alone' context unless it's got another motive (these things might happen in the changing rooms among a group of mates, then it's just fun). Are you supposed to admire him for taking out his penis in front of you who have already seen it? What's brave about that? It's a dare for you to get your penis out too, and you have occasionally done so, so you're now wondering: what comes next? There are two things you can do and they could go either way.
Next time you go and shower, take all your clothes into the bathroom and come out dressed, thus not allowing for any penis flicking or any underwear / crotch watching, if that makes you uncomfortable, or if you don't have time for that sort of thing.
The other suggestion is coming out of the shower wrapped up in just your towel and doing all the dressing in your room in front of him. You can't be that shy that you won't want to show him your naked body. See if he averts his eyes or if he watches you while you dress. I think he's been thrusting his own genitals in your face sufficiently for you to understand that he might be interested in some fooling around. So, OK, maybe he's curious too, and maybe he's also more interested in you, romantically, than he will admit. I don't think straight people can turn gay, if there is no gay feeling elsewhere. Maybe if it's just curiosity, you can get it out of your systems, play around with each other's bodies a bit, feeling another man's penis is interesting and warm, but ultimately only you and he will know if this isn't your thing. Maybe dabbling in a bit of man on man sex, you'll be able to differentiate with what goes on when you're with your girlfriends. I mean, have you both already had full-on sex with your girlfriends? Will there be a point of comparison possible?
How far would you be willing to go with him? Massage, kissing, caressing, stroking, mutual masturbation, fellatio, more? If you are thinking about going pretty far, then just make sure you have the right amunition to deal with it in a sensitive and sensible manner. So make sure you understand his limits and you state your own. Do you want to watch or make one another climax? As far as the sensitive and sensible manner is concerned, you might need lubrication and you might need condoms (just saying -- maybe you don't even want to go that far, or maybe you'll escalate your way there but maybe not in one go).
Good luck with finding out whatever it is he wants from you, and enjoying it, if you get what YOU want.