I have a history of having foreign significant others visit me in the US repeatedly for several weeks at a time. None of them were ever denied admission to the US even as they all said they were visiting a significant other (i.e. me) during their vacation/holidays.

While disclosure about visiting a significant other does rather frequently lead to an increase in questions being asked by CBP, being questioned and then denied entry by the CBP is not anywhere close to the most common outcome for foreign citizens visiting their significant others in the US -- it's relatively infrequent.

A lot of US citizens involved with EU or Canadian citizens have their EU or Canadian citizen SOs visit the US for 4-6 weeks at a stretch during school (if a student) or vacation from work. Most are admitted without a major issue.

My take is that the more you worry about these situations that mostly go over well enough for most others in the same circumstance, the more likely you are to end up with a problem.

I'd keep it simple: "I'm visiting my boyfriend/girlfriend during my vacation." For any questions arising after that, I'd suggest keeping it simple and sticking to the truth. The less you worry about, the easier it seems to go.