That's pathetic, bullying is bad at any age but to bully someone when you're an adult, it's just vile. I hope you can realise it's THEIR problem. There's absolutely nothing wrong with a 31 year old being single... whether you chose to be or whether it's just because of circumstances. I'm 24 and single and there is NOTHING wrong with it. My dad is 54 and single and there's nothing wrong with that too.
It's just very small minded of your work colleagues to a) put emphasis on a non-issue and b) try to degrade you by name calling.

I think somebody mentioned that it is grounds for sexual harrassment. If this continues I would defintely go to your manager, or whoever, and explain what is going on. It's not right anywhere but in the work place it's illegal... if they're such vile people they need to bully another human being, they deserve to be punished.