Because I Want To by Grace R. Duncan

Omega Adam and Alpha Matt have been best friends all their lives. Matt has been there for Adam from his first heat. They’ve never been apart and Adam has never considered being with another person. So, it confuses Matt when Adam refuses to allow Matt to bite and pair with him.
Adam has seen for himself what happens when an alpha leaves his paired omega, when his mother went through the excruciating pain of a new alpha bite. Knowing his best friend as well as he does, he knows that if Matt ever bit and paired with him, Matt would never leave and consign Adam to the pain of a new bite or a life without sex. But Adam knows that it wouldn’t matter how Matt felt, he’d stay even if he felt trapped by the situation, making them both miserable and hurting Matt badly.
When Adam finds himself pregnant with Matt’s baby, he panics. All his attempts to keep Matt from getting trapped have been useless. With no idea what to do, he ends up hurting Matt anyway. If he doesn’t come to terms soon with his fears, he could find himself completely alone and losing the only person he’s ever loved.