The term sexual partner is usually applied to consensual sexual relations, not to those that are forced or result from duress, as in the case of rape or bestiality. In those cases, one party is typically called the perpetrator, and the person against whom the sex act is imposed is called a victim, or by similar terminology.
A sexual partner may or may not have equal power during a sexual activity; such sexual "roles" (which can sometimes be determined by personality type but can also be determined by a conscious choice by the individual to "assume" a particular "role") typically fall under various dominance/submission (D/s) categories.
Generally, a sexual partner is one with whom one is engaging in a sexual activity, or one with whom one regularly engages in such activity and expects to do so again in the future.
An ex-sexual partner is a person with whom the other partner does not intend to engage in any further sexual activity; while a prospective sexual partner is one with whom the person has not previously engaged in any sexual activity, but desires.