Straight works better, but it expresses a spectrum. This is often described as being oriented close to you while that is described farther. Straights are fucking what's father away from their gender.
In theory: Bisexuality is attraction to more than one gender, while pansexuality is attraction regardless of gender. For example a bisexual person could be attracted to women and nb people but not to men.
In practice: Mostly synonyms, some years ago 'pansexual' was kind of a buzzword to mean 'inclusive bisexual' (as in can also be attracted to trans and nb people), but I haven't met a bi person that wasn't 'inclusive' or that wasn't attracted to a particular gender like the example above.
The old definition of bisexual is attracted to both men and women. The more correct definition is attracted to more than one gender. So saying "fuck him and fuck her" is limiting to two specific genders and not reflective of what bisexual is.
Bisexuals aren't exclusively attracted to men and women, nonbinary and agender folks are largely included. Common interpretation of the bi=2 prefex is including 1) same- (homo) and 2) different- (hetero) gender sexuality. Colloquially the difference between bi and pan is usually described as bisexuality allowing for preference for certain genders but not exclusively so, while pansexuality is attraction regardless of gender. A given bisexual might be attracted to twice as many women as men, but that doesn't mean their attraction to those men isn't real, you know? Meanwhile a pansexual could have attraction to the exact same people but wouldn't ascribe it to gender, just individuals. Largely the difference between the two is therefore personal preference in how someone experiences their sexuality.