There's no rule that says you have to keep doing this - or even that says you have to forgive your spouse for an affair. Only you can know what's right for you.
I will make a few observations, both pro and con:
Pro - - he just got back from freakin' Afghanistan. He's probably been through things you can't even imagine. His affair may just be the symptom of depression, PTSD, post-concussion brain damage, malaria-drug side effects (although I think the military stopped using mefloquine, right?) or any number of other medical/psychiatric effects of war. This may be the "in sickness and in health" part of the marriage. EVEN IF he goes off with OW and never comes back, you might consider finding it in your heart to forgive him if you think of it as a consequence of the war. And you might, for now, consider giving him a little more time to figure things out.
Con: - I see that he has another older son - so I assume you are a second wife (or at least second serious relationship?). If so - and considering that you have only been married 3 years and he is already cheating - it is possible that he just has a character disorder. If you look back honestly (not just through the lens of today's events) were there big red flags that you ignored about him when you met him? Does he have a history of bailing on relationships, or a history of infidelity in past relationships? If so, then you might be right about cutting your losses now - repeat cheaters seldom change (although it's not impossible).
Pro: - you're just about to have a baby. You have to consider hormones in any decisions you might make today. You might consider postponing ANY major decisions until a few months after the baby is born, just to avoid making any rash choices out of hormone hell.
Con: - being the strong woman who won't put up with any cr@p and is perfectly willing to move on without him....is sometimes the shock that wakes them back up. You sure can't count on it - it's very risky as a tactic. But if you're really feeling that done, then sometimes letting him feel the full real-life effects of his affair can cause him to change his mind before you get too far away to ever return.
Step back for a minute and try to see what's really going on here. Is he a brain-damaged PTSD veteran who needs some slack? Are you a self-centered harridan who needs to change her ways? Or is he a serial cheater who is just repeating the pattern that you ignored when you married him?
