Men, just like women, have different opinions and reasons.

but under what circumstances would a should a man agree or disagree with an abortion?

Some men do and some men don't. The reasons are about the same reasons as women give, which are wide and varied.

1) As long as a relationship exists between two persons, some decisions have to be taken in common. If there is a disagreement about the decision to be taken, the relationship could break. 2) Normally, in the case of an abortion, it is the woman who will primarily take the decision. If she decides to have an abortion, but the man would rather have her keep the child, he could decide to leave her. Also, if the woman decides to keep the child, but the man does not agree with this, it could also be a reason for him to leave her. For this reason, it is much better when an agreement can be reached. This usually will not apply if the relationship is already forever broken. 3) Most abortions happen when contraception has failed. It was not planned to have child, and in this situation, the man could change his mind and accept the child, or remain by the original rejection. An abortion could also be decided if the woman has been raped by someone (else than her partner), or if it would become a child of a former partner. There could also be medical indications. Maybe the partners are just friends with benefits and don't envisage living together. Maybe one of them is married to another person. Maybe they just cannot afford a child, or don't want to lose their liberty. Maybe they are too young. Or the child would be the product of child abuse or incest.