Aren't they bisexual then?
I don't think so. As far as I know they (they live in a very conservative totalitarian European country and are professionals) married to have a family, for social conformity, for professional reasons and for family expectations. This is far more common than you'd think. They both have other relationships. Whether or not there's ever been anything between them is their business, however if they're not, they're in the right line of work to do a turkey baster jobbie.
a different sexual orientation
You say it well, however I'd add that it isn't so "different". Kinsey's research in the 50s and 60s showed that about 50% of people were either absolutely at one end of the spectrum or the other and 50% were on a sliding scale in between.
Anthropologists suggest that 85% of North African men (stats for women would be near impossible to get) have had frequent same-sex relations. Almost none of them, would identify as gay or bi.
In the late 80s in provincial England, you saw the same few dozen people in the bars pretty well every time you went. Then suddenly in the mid 90s, the scene exploded, thousands out on a Saturday night. All 20 something. Where were their older brothers/sisters?