First it is "social media and electronic gaming", then the reasons "may also include stress of juggling work and intimate relationships, as well as the prevalence of other forms of solo entertainment."
But then, they notice that "[s]exual activity was largely unchanged among unmarried women, along with no notable decline among gay men, researchers reported." So everything said before seems kind of void anyway.
Obviously, the reasons must lie somewhere else. However, this is not explicitly stated.
Now they continue with huge jump: From sexual activities to relationships: "Given a preference for men of higher socioeconomic status and the larger number of college-educated women than men, some young men may find it difficult to form heterosexual relationships"
Well, maybe. But how does that connect to the first part?