ok...here is my opinion...dont get fixed if you have doupts and if he doesnt make his kids obey you that may end up being a problem so u really dont need to be fixed if you may not be with him forever...u can always have a 5 year birth control.
If you have any doubts at all, don't do it. You never know where life is going to take you, whether it's staying with your current bf or moving on to someone else, and you may find someone else down the road if that's what you want, and want to have kids. My sister had a boy and a girl with someone she wasn't really 100% happy with, and ended up getting her tubes tied at 24, and she is dating someone else for almost 2 years, and wants a baby with him soo bad but can't, because she's fixed, and he really wants one. It's too permanent of a decision to make at 21, I'm 21, second child, one boy, one girl, and married, STILL not getting my tubes tied. If my husband dies, and I meet someone else, if we split up, if we decide we want another 3-4 years down the road, I can. My husband is all for waiting, he doesn't want me to get my tubes tied either, so REALLY think it over! You sound too hesitant to go through with something so permanent, and with great reason, I love babies, and only 2 seems like not enough for me!