Again, because you seem to think that bisexuals are only able to romance other bisexuals. Straight men have two options (Cassandra and Josephine), just like everyone else with the exception of straight females. You don't have "almost nothing" because you have two options, which coincidentally is the same amount of options that you had in the last two DA games as well.
ok lets make it that way if you insist
Straight males
1)Cassandra-female, human, templar/seker, pro Orlais and pro Chantry
2)Josephine-female, human, pro Orlais and pro Chantry, old girlfriend of Lelianna
Straight Females
1)Solas-male, elf, apostage mage without connection with Circles and mages rebellion
2)Blackwall-male, human, ex grey warden constable, neutral sided
3)Cullen-male, human, templar/seker, pro Orlais and pro Chantry
4)Iron Bull-male, kosith, Qunari soldier-spy, pro Qunari
Lesbians
1)Sera-female, elf, city elf, pro city elfs
2)Josephine-female, human, pro Orlais and pro Chantry, old girlfriend of Lelianna
Gays
1)Dorian-male, human, criminal and exile with shame from Tevinter who was a member of venatory
2)Iron Bull-male, kosith, Qunari soldier-spy, pro Qunari
Bisexuals
1)Iron Bull-male, kosith, Qunari soldier-spy, pro Qunari
2)Dorian-male, human, criminal and exile with shame from Tevinter who was a member of venatory
2)Josephine-female, human, pro Orlais and pro Chantry, old girlfriend of Lelianna
2)Sera-female, elf, city elf, pro city elfs