I accept the 'old bachelor' idea about Bilbo. He's the kind of person (or Hobbit) who's considered a little odd, but he accepts it with a shrug, minds his own business, and hangs out with his cousins rather than a wife.
Frodo was strongly influenced by Bilbo, so his bacherlorhood likely started there. In addition, he seemed a bit more introverted than the other Hobbits, and there's the strong possibility he simply wasn't interested in marriage. That's how I'm wired - I'm just not interested. Maybe I'll meet someone in the future, but I'll let that take care of itself. For now, a boyfriend would serve no purpose.