Thanks to everyone who has replied (and sorry for reviving the topic, being a bit busy as of late), judging from the majority of comments from this thread, I'd say that the diet plays a crucial role in sex.
I didn't tell you guys that its not sex that's the problem (I'm stilla virgin), I'm talking about erection problems. I don't really want to think I have Erection Dysfunction problems, because I'm only 18 and I do get erections, but not at the instant of watching a porno movie or something (my girlfriend knows that I watch it and does not find it a problem, she said its normal for guys to watch it), my sexual thoughts have decreased as of late and thereforeeee, I'd need to stimulate my penis by rubbing the head to get it up (Sorry about the details), I do cum when I masterbate, but its just that I need to rub the penis in order for it to go up, and other times during make out sessions with my girl it goes up because I'm being aroused, but not really really hard, just good enough. lolz
However, I do see my diet as a major problem to my sex-drive and testosterone decrease levels. I've been dining out alot, for the past 2 to 3 weeks (this problem has happened for 2 weeks now), I've been only eating dinner at home for like twice a week (my dinner consists of rice, usually fish and meat and veges... the usual Asian dinner). I've also been eating a hell of a lot of noodles lately, two packets in one meal, and sometimes 4 packets a day... I put both packets of flavoruing and I also ate a lot of KFC chicken and McDonalds. Exercise has also decreased as of these few months.
I guess, its just my testosterone problem, I would need to change my diet and exercise more. I'd hate to think its Impotency because the thought of that at a young age scared the hell out of me.