POSSIBILITIES --
Traps:
You and he have already exchanged some risque texts.
Now there are some texts not sent by you, band they are risque too, only much moreso.
If he wished to assert that you were the sender of those two texts, it would be believable.
He has gone to some trouble to make this scenario (that you are the sender) look believable.
Truthfulness/gaslighting:
He has flown a story by you before.
You didn't quite believe it but you carried on with the wedding plans and are now pregnant.
You nevertheless were impelled by something to look at his spare phone.
Fetish/sexual interest:
He is capable of compartmentalising his life in this area.
He is capable of dividing you into madonna/whore personae. Not good when you are about to become physically the epitomy of the 'madonna'.
He is not as he seems at all in the sexual area. He is not the person he is presenting to you.
MUSINGS:
He may have seriously cold feet and the upcoming wedding and now the pregnancy have terrified him but he feels he can't get out of the situation for various reasons of his own. He is panicking.
He may have serious psychosexual problems, may not have integrated his sexuality into his personality.
-- or there may be a bit of both.
To have sent both texts from home is a breach of the boundaries of your shared home. He was mentally somewhere else when he sent them.
To have sent one of those texts when you were away on a wedding related activity may indicate the extent he is baulking at the wedding/marriage/responsibility/fatherhood of his own child...
-- he is not ready for the wedding. He is not fully committed to the relationship. He is holding something back.
Most creepy scenarios (but nevertheless possible):
He may be planning to use the texts to threaten you with -- 'if you don't do X with Y and Z I will show the texts to A, B and C...' How embarrassed would you be to see them broadcast?
He is leading a double life that involves porn and a lot of fantasy and as has been repeated by Sakura and Dignified, et al, you are a mere cipher. The contradiction between reality (impending baby and wedding) and fantasy (porn and the threesome thing) are two features in his life that he can't integrate.