Why do guys change their minds about doing it or not doing it?

First, he has no business bringing up that old business of yours. He might bring it up as a way of putting a guilt trip on you, and that's wrong of him.

As far as pulling way and then wanting it, it sounds like he has problems. You can't keep going through this, you deserve better. I hope you will gather the courage to lay down some rules with him to change this, or else you may need to stop seeing him.

Men are fickle and sometimes selfish without realizing it when it comes to sex. I mean that the issues that pop up are they:
1. virgin
2. Uncommitted in Relationship
3. Religion
4. Dissatisfied/Low Sex Drive
5. Morals Complex
6. Stress/Anxieties
7. Mental Afflictions (Psyc)

and it does seem like your boyfriend is being manipulative and wants it his way... I would get him to talk about it before initiating any more intimate contact and explain to him your frustrations. He might not even know that he is doing this and just needs a good wake up call from you to bring him around. Or else have him see a professional adviser to keep this relationship positive... I hope that this will be resolved soon for both of your sakes and do it sooner then later, as it will only continue if you allow it to... I know that everything will work out but it takes both of you to be honest and communicate on this subject and get it resolved!!