There are only 24 hours in a day. If you pick up a new significant other (we’ve had threads asking the reverse question, and the answer’s the same either way) it is a simple matter of mathematics that if you start spending time with a new lover, you have less time for other things. It is furthermore common sense that given that much of the time you spend on other things is difficult to change, like your job, you must sacrifice some time from other leisure and social activities.
I had the opposite happen, I had a friend who dropped me when I got a boyfriend. Actually, it was kind of a mutual dropping. She’d decided for no good reason she didn’t like him when he’d been nothing but nice to her. I see her on FB sometimes, but we’re not really friends anymore.