PvdS said that in an interview in a Dutch language newspaper. The translation has been disputed in that it could mean something less than actual sexual relations (in the Clinton sense)(that is just making out).
They said they had sex at the beginning, and then changed it to say that they were just "making out". I guess the dutch words for sex and making out are either the same or similar and you must understand the context before you can determine the accurate meaning.
I read they used the word "vrijen" which must loosely translate to "fooling around" because they said it can mean anything from kissing to intercourse.