Is it obligatory to fulfill a vow of fasting two consecutive months for someone who made such a vow to avoid masturbation, but then emitted semen as a result of pressure on the lower abdomen and not from manual masturbation?

Masturbation is forbidden. Ejaculation due to mere thoughts and imagination is not considered masturbation. As for the vow mentioned, it is a vow made in a state of rage and anger. Its maker is given the choice between fulfilling the vow or atoning for it with an expiation for an oath (kaffarat yamin). Therefore, if you break your vow, you have the choice between fasting as you vowed or performing an expiation for an oath, and you must repent to Allah the Almighty.