For quite a while, I've been facing a problem where when I pass stool forcefully, it causes the emittion of semen. Now, I'm sure that ghusl becomes wajib on me, but because the semen doesn't get in contact with my clothes, do I have to change them everytime?

First of all, are you sure what you see is semen?
Most probably, this is wadi, not semen, which is clean, and neither needs cleaning nor ghusl.
If you are 100% sure that it is semen, then you need to do ghusl, but you do not need to change or wash your clothes if the semen did not contact them.