Is it possible to get STDs if youve only had sex with one guy and the guy doesn't have stds?

Certainly. I would suggest that one of your partners is infected. If he is the only one you have had sex with, then he must, by definition, have it.
No. You will not get std if your partner has never had std. Your partner is not being honest if you have developed std, have only been one guy, and he denies having had std. Some man may have been treated for or even had mild untreated case of herpes in the past and either believe they never had it or think it occurred so long ago that they are safe. So if std is herpes, or another std he had it.