If a person is married can they “worship” at a temple of Uleria, or is this seen as a breach of the marriage vows?
Of course they can.
In general, the legal/magical framework for marriage is going to be driven by the cults involved. So for most of the Orlanthi, Ernalda is going to establish the framework for marriage. That puts the woman at the center, creates year marriages and requires that the husband and wife (and their kin) make tangible promises to each other (husband to provide protection and aid, wife to provide blessings of life and support). Remember that marriage is largely a matter of social responsibility. The goal and focus of all marriages was intended to be reproduction and/or political alliances.
Orlanth’s big contribution is to add honor to the mix and sets this as the rules for his big and sprawling network of companions, kin, and followers. Neither god expects life-making powers to be exclusive to marriage – and men and women can take lovers or concubines (or even additional spouses) as long as they can still do what they promised to do. Noble often take more than one spouse. Divorce can be initiated by either party, but there may be legal consequences of divorce (property, kin relationship, etc) rather than just letting the year expire.
Clans and kinship groups make alliances with other kinship groups. You might only be a free farmer with the Orlmarth clan, but an alliance with an Ernaldori family can provide additional support, allies, protection, legal assistance, access to temples, wealth, etc. As we learned in the Seattle Farmers games, those free farmer families are every bit as political as the noble kinship groups. And they need allies even more.
Earthly concepts of “love” and “loyalty” apply of course, but for most ancient cultures marriage is a fundamentally social and group activity. Romantic love leading to marriage of course happens – and Orlanth Adventurous certainly encourages that more than say Ernalda, Yelmalio, Yelm, etc – but that is not the norm. The norm is to have a marriage to another kinship group that has been arranged by or at least in consultation with your kin.