Does the ability of a Space Marine to have sex really matter in the game? Seriously, I see no reason why it would ever even become an issue unless part of the mission is to go and **** the enemy to death.
And besides, what makes you think they are all straight? If history is any indicator, the greatest warriors were the Theban Sacred Band, 50 gay couples who were the ultimate warriors of the ancient world (in some opinions, Alexander thought so). They died to the last man from overwhelming numbers.
Perhaps the homosexual chapter motto can be "Never leave your brothers behind."