I also totally agree with LiyoS's sound advice.
What I would add is that your mom has, to the best of her ability, handled this the best way she can. She sees her daughter growing close to her bf and she is concerned about the potential consequences. Her mom could have sat back and kept her fingers crossed and if a 'mistake' happened go crazy at that point and kick you out etc. Instead, she sat you down and explained her concern and has asked that you respect yourself and not give in to sexual temptation at such an early age.
She treated you respectfully because, I promise you, that was a conversation she didn't want to have anymore than you did. She has set her boundaries and expectation and let you know the repercussion if you chose otherwise.
Of course, it feels like the next logical step but it doesn't have to be. If your bf cares about you as a human being he'll be fine just being your bf without sex. Yes, he would probably want to go further but will respect a 'no' decision. IF, however, he's found a girl that he thinks will be easy to get sex from he may move on if you don't have sex with him....... in which case YAY! - he wouldn't have ever been a long term prospect anyway. You know your boyfriend does he want a relationship that can build over time or is he pressuring you.
Another potential risk of having sex too early is, besides STI and, of course, pregnancy, is it is much easier to say yes the next time. IF you and your bf have sex but don't stay together then the next guy to come along you'll say 'yes' to a lot quicker. That can lead to ONS or just dating a string of guys who are happy to 'get what they can' and move on. No, not all guys are like that, I agree, but there are enough that will choose to be like that. Eventually this will challenge your self-esteem.
No one is saying 'no sex until you're married and no marriage before age 30' but, again I agree with the above poster that you should wait. Respect your mom and most importantly respect yourself! You are blessed to have a mom who loves you enough to confront the situation. .