I would recommend that you analyze your own reasoning behind wanting to withhold sex, instead of finding ways to persuade your bf to not want to have sex with you.
Is it very important to you to be married before having sex? If so, is it a religious belief or just general insecurity? if it's religious, fair enough - but then you should just calmly tell your boyfriend that due to your personal beliefs you wish to remain virgin until you are married. If he knows you and your convictions, and loves you, I am sure that he will understand. And if he doesn't, you are clearly dating the wrong guy anyway.
It seems to me like you have reasons rooted in personal insecurity though. You mentioned that "may be that I am afraid of being cheated on because men usually lose the interest after getting that close to a girl. "
This, to me, sounds like the words of an insecure person who has learned how sex and relationships work from sit-coms. I am not trying to be hurtful, I am just saying that this belief doesn't really have any ground in real life, unless you count one night stands. Your relationship has already been ongoing for such a long time, that it's not likely your relationship would end because he would 'tire' of you after 'putting out'.
Instead, to me, it seems like you are withholding sex as a means of controlling the relationship (=stopping him from losing interest), which is not a very healthy thing. I recommend that you analyze your own attitude towards sex first - are you afraid of it? Do you feel like you will be 'used' somehow? If this is the case, I recommend that you do not have sex with him now, but instead try to get to the core of your own insecurities first.
Also, I recommend that you analyze the relationship you're in. Why would he tire of you after finally getting you to have sex with him? Are you already sensing that the only thing between you and a break-up is his hopes of getting sex? Or is you relationship in balance, but you still fear having sex?
As a woman, I can tell you that you have nothing to be insecure about. You are getting as much out of the physical relationship as he is.
Religious reasons aside, I am also questioning why would you also want to marry a man whom you don't know intimately yet? By experience, I can tell you that sometimes you find out something unexpected and unpleasant about your partner and their sexual preferences after sleeping with them. IMO, it's better to find out sooner than after tying yourself down for the rest of your life. Also, being married doesn't necessarily mean he won't ever tire of you. Just look at the 'cheating' and 'divorce' sections on this web page.
Basically, I think you may have problems with your own insecurities, which you should address before getting intimate. However, you might want to consider a compromise (not going all the way, but fooling about, if you catch my drift) if you are absolutely not willing to lose your virginity before marriage.