The thing that immediately occurs to me is that the 30k mark of a 100k novel is right around the end of Act 1/start of Act 2. (Typically Act 1 is c. 25% of a novel).
I suspect that what's happening to your story at this point is that the inevitable rhythms of the 3-Act structure are kicking in.
Act 1: Ordinary World...Boy & Girl don't know each other at first...then they meet & BAM! We all know they have to be together by the end. This leads to transition into...
Act 2: Special World...Boy & Girl are now on journey (they are in the "Special World" of blossoming romance, even if it hasn't happened yet) that will inevitably end up with them together. But!!!...Along the way there is drama, conflict, problems, tensions, emotion & action all working to try and thwart their coming together. This is where the main drama of the story happens.
And so, it would be perfectly natural around 30% of the way into your book that Boy & Girl sleep with other people, all of whom will end up being wrong for them. Why? Because the ONLY person who is right for them is each other. But we (the reader) need to see the drama of those other "relationships" NOT working to understand why Girl & Boy DO work.
In fact, the start of Act 2 is sometimes called the "Friends & Enemies" section. Here, you'll see:
> Girl's friends encouraging & helping her to get together with boy
> Boy's friends doing the same
> Fake friends trying to keep them apart (For her, that's the bitchy mean girl in her circle who encourages her to go to bars & sleep around & "forget about that loser". For him, it's the rich jock buddy who he goes out with to try and score notches on the bedpost. These fake friends thwart Girl & Boy getting together).
> Enemies: These are the random guys she sleeps with - they perform the function of showing what the Boy she loves ISN'T. Similarly, he meets up with girls who just want his money or commitment or marriage...while all along he just wants the great girl he met in Act 1.
> Other enemies: For example, his boss might have Boy working so hard that Boy has no time to chase Girl. For her, her Mother might be in her ear telling her to marry that nice accountant on the 2nd floor. All these people are obstacles to girl and boy getting together.
..........
So I think this is what is happening. You are just in the normal, natural rhythms of the 3-Act structure.