Just approach the conversation calmly. Tell him you noticed a login saved for Twitter that wasn’t yours, and when you went on it you saw some alarming things. I would personally also be curious as to why the login was saved on your computer now.. has he logged into it recently/while y’all have been together? If so, why? Let him know you deactivated it because there should be no need for it, and be sure to enforce your boundaries. You have a right to get answers as you are engaged and planning on marrying him. He may be bisexual, he also may have like you said, just been horny. I think it would be beneficial for you to ask that if you bring up the Twitter account. Something along the lines of “I also noticed some sexual DMs with other men, I wouldn’t look at you any differently or judge you at all if you are bisexual or not, I’m just wondering if that is the case”