It's not irrational and don't beat yourself up over thinking that it is. For your first time, it's good that you are thinking about it, too many women don't and the consequences aren't what you hoped for.
Do some more reading, check out some sites like
http://www.plannedparenthood.com/ and even Ivillage for first time sex. If it helps you, go visit a planned parenthood clinic and talk to someone there. It's one of the reasons why they are there.;
I'm sure your boyfriend is understanding as well, which is great, talk to him of your concerns and research it together, more education couldn't hurt.
Your first time should be special and there shouldn't be fear or doubts involved. Trust your instincts, they're rarely wrong. and I'm sure the ladies here will do whatever they can to help you out (I know I'm pretty good at listening)