A heterosexually active population is exposed to two competing strains or two distinct sexually transmitted pathogens.

It is assumed that a host cannot be invaded simultaneously by both disease agents and that when symptoms appear, a function of the pathogen or strain virulence, individuals recover.

We conclude that in a behaviorally and genetically homogeneous population coexistence is not possible except under very special circumstances.

The mathematical qualitative analysis of our model is complete ; that is, we provide the global stability analysis of the stationary states.

We conclude this manuscript with two extensions.

The first allows for the possibility that a host may face multiple competing strains, while the second looks at the effects on coexistence of the host's age of infection when two strains compete for the same host.