From Wikipedia article on Diomedes:
Diomedes, in turn, married Aegialeus's daughter Aegialia when he returned from battle. He was then appointed as the King of Argos and thus became one of the most powerful rulers of Hellas at such a young age.
(the battle referred to in the first sentence is battle at Glisas, during Epigoni war).
Later on, as explanation of why he joined the Achaean expedition to Troy, it says:
Diomedes became one of the suitors of Helen and, as such, he was bound by the oath of Tyndareus, which established that all the suitors would defend and protect the man who was chosen as Helen's husband against any wrong done against him in regard to his marriage. Accordingly, when the seducer Paris stole Menelaus' wife, all those who had sworn the oath were summoned by Agamemnon (Menelaus’ brother), so that they would join the coalition that was to sail from Aulis to Troy in order to demand the restoration of Helen and the Spartan property that was stolen.
How is it possible that a married man was allowed to be one of the suitors of Helen? (either by his wife, or - from Ancient Greek viewpoint - more importantly, by Tyndareus)
I considered that Wikipedia's narrative merely reversed the chronological order of the two events, but all descriptions I saw of the suitors of Helen was that they were kings/princes, and Diomedes only became a ruler post-marriage.