Ask to see a lectionary with selections for nuptial Masses. (You can use these for a wedding without a Mass, too, of course.)
Nobody but a narcissist goes to a Catholic wedding and expects Jesus Christ to be washed out of it. Don’t even try to whitewash that. It can’t be done, and it shouldn’t. Besides, many (if not most) non-believers agree with Mark Twain: There has been only one Christian. They caught him and crucified him–early. If you restrict yourself to what the Lord has to say that can be heard by any person of good will, you will find there is plenty.
The truth is that most of Scriptures offend no person of good will. People don’t like doing what Scriptures say, but they find it pretty hard to quarrel with 90% of the theory. If you stay away from the 10% that requires a more complicated explanation to those outside the faith than what a homily allows for–like what submission means in marriage or the conditions under which civil divorce is permissible–you will still find a lot that the Holy Spirit has to say about marriage, its joys, its demands, and perhaps what marriage means to everyone present. Even if you just choose “what does love ask of us?” as your theme, you will find some very deep and demanding words in Scriptures.
----more advanced considerations:
A lot of Catholics do not realize that the Liturgy of the Word ideally has a clear connection between the readings, or at least between the Hebrew Scripture, the Psalm, and the Gospel. Allow me to explain.
Go into a Sunday missal and read the Psalm, particularly the response. Then read the Gospel. Do you see where the response anticipated a main theme in the Gospel? Now go back to the first reading. Do you see where there is a theme or image from the Gospel in the first reading?
In the Liturgy of the Word, when the readings are chosen by a liturgist instead of an uninformed layperson, there is normally this progression: The first reading is a sort of a foreshadowing of what will come in fullness in the Gospel. Then, in the responsorial psalm, the people sing out a kind of a “ooo! ooo! We see what’s coming! Praise God!” This is a song of recognition of what the Holy Spirit is getting at, the direction that the Word is going. Then, in the Gospel the message comes through in its fullness.
Therefore, I’d choose the Gospel reading, then a first reading that speaks to the theme of the Gospel or an image of marriage relevant to the theme, then the psalm and response that connect the two. The connection in the second reading does not need to be obvious: it could serve as a reinforcement, a broadening, or a contrast of the theme.
By the way, when we got married, we had a wedding booklet that included “A Road Map to a Catholic Wedding” in which we explained that not a few non-Catholics feel like they’re taking a test covering a class they’ve never attended when they go to a Catholic church. We explained that the sit/stand/kneel events would be clearly marked in the booklet, that it was OK to sit when everyone else was kneeling (no promises if you stand and block someone else’s view) and so on. We had Mass, so we also explained what was going on in the various parts of the Mass and the wedding ceremony and the business about having to be Catholic to go to Holy Communion. We even told people how to find the bathrooms, and that it was OK to leave Mass and come back.
We got wildly enthusiastic response from the booklet, from Catholics and non-Catholics alike. If you have the resources to do something like that, I highly recommend it.
This was the Gospel we chose, although most lists of readings for a nuptial Mass don’t include it: They came to Capernaum and, once inside the house, he began to ask them, “What were you arguing about on the way?” But they remained silent. They had been discussing among themselves on the way who was the greatest. Then he sat down, called the Twelve, and said to them, “If anyone wishes to be first, he shall be the last of all and the servant of all.” Taking a child he placed it in their midst, and putting his arms around it he said to them, “Whoever receives one child such as this in my name, receives me; and whoever receives me, receives not me but the one who sent me.” Mark 9:33-38