For both nobles and commoners marriages are a contract that expects political gain, whether the marriage is for love or not. Marrying for love is a little less common than marrying for politics but over all it is pretty evenly split. What gender the couple is has no bearing on whether a match is made. Same sex, opposite sexes, and any sex in between is considered acceptable candidates for a marriage. Love can be an important factor for both noble and commoners but it is not often the priority for a match. Commoners have a little more leeway in this aspect as it is quite acceptable for a commoner to marry purely for love but often even their matches are used for some sort of political gain.
A marriage can be forced by the heads of a family (IC it can be, never OOC), though, it is not commonly done, as it can breed resentment that can spill out of the marriage and sour relations between families. The preferred marriage match is one that both parties are willing and not coerced into it but if the gains for both parties is good enough, a forced match can happen.
Nobles can absolutely fall in love with a commoner but marrying one is never acceptable. Commoners can not, and will not, be elevated to nobility with the exception of Children of Gods. However, this does not mean that the noble can not take them on as a consort (+news consorts). Having a Consort is an accepted thing in marriages, love match marriages can be a little greyer in that aspect, though.
Commoner and nobles alike have a contract written for their marriage. It is generally witnessed by the couple, a priest, family heads, and a member of the royal family (can be NPCs). A marriage contract is really not a very private affair. The details that go into the contract is up to the people involved in the marriage. The major thing that is noted is what the gains for the families are. Noble or Commoner contracts alike have exchanges written into them. This means that if a noble marries into a family, a member of that family is married out to the contracted family. This is also the same for Commoners but with them it is likely that part will be glossed over and treated as an obligation written in but not always followed.
OOC: To note, while political marriages are common we expect players to be mindful that OOCy the people being married should get along or enjoy RP with each other. Marriages will NOT be forced OOC but they can be forced on an IC level.
Staff will also be allowing marriages to NPCs if that is something preferred, as there are people who prefer not to have a PC spouse either due to time restraints or personal preference. HOWEVER, to get married to an NPC staff will require the same expectations one would get from PC marriages. Staff will also not be creating your spouse as part of their NPC roster unless they started that way.