Your scenario does not account for the reality of sex. It is more than merely a good time - it is intimacy, emotionality, security, comfort, etc. These are things humans need and want. This is part of our nature - Avoiding sex isnt a reasonable expectation for sexual beings.
Secondly, pregnancy isn't a burden everyone can endure. Some might lack stability, resources, physical ability, maturity, committment, etc. Forcing a burden in spite of the inability to carry it is cruel and irresponsible.
This is where loose definitions cause problems. The inevitable response will be something along the lines of 'Well, it is a human being - it is not a burden that can be denied', however when an adult woman, a fertilized egg, and a tumor might all qualify under an overly broad definition of human being, this argument can't be taken seriously. A human being isn't merely determined by human DNA or the potential to be a person.
Finally (bringing us back to the OP), a person has the right to say, 'no - I do not consent to having my body used that way', and that my friend, is the end of the story.
Tl;Dr- Your definitions are too broad and your scenario is too simplistic to draw any meaningful conclusions regarding abortion.