Now, I want you to pay attention. Jesus said "has committed ADULTERY". Adultery is a synonym for an affair which is reserved only for folks that are already married. If you are single, you CANNOT commit adultery.
If you are single and you have sex with another single person, then you have committed FORNICATION because it is outside of marriage. Adultery and fornication are NOT synonymous. Jesus does not condemn anyone that is single from looking at another single woman with lust, as long as you don't act on it. He does, however, condemn married men from looking at women that are not their wives.
Now, before you begin your rebuttal, if we go b y your line of thinking, then it is also a sin to look at your spouse with lust. Silly right? Well, that not true because you can't commit ADULTERY with your wife, right? Well, single people can't commit adultery either. Case closed.