Ok. It sounds like to go ahead with this kind of conversation a rough plan might be a useful starting point? Obviously you know this person best, so I'm going to throw out a few ideas and we can go from there. Unfortunately, there are no guaranteed results or quick fixes but some approaches to take can be:
1) Appeal to their better nature: "C'mon, I know you're better than that", "You know, I always took you for a tolerant person, why would you say a thing like that?", "One of my favourite things about you is how you just take people as they are, don't ruin that"
These kind of things can be little comments that you just put out there and change topic, trying to nudge a behaviour change or can lead into a wider conversation if you think you can actually change their mind.* Are there any examples of times they've been a good friend/ally that you can draw on?
2) Ask them questions. These can be questions to try and understand what they're saying, or questions that challenge their ideas, or both. "Do you think that you can always tell someone's sexuality by looking at them?" "Always?", "Why is that even useful/important?", "Why would being straight or gay be more likely than being bisexual?" "How much do you think sexuality changes over a lifetime?"
Start where they're at, and then ask questions. The theory is you make space for them to think through what they're doing and find the flaws in their ideas for themselves, but it means doing some listening.
3) Give your perspective. "I've always thought that....", "In fact, I saw this study that said...", "You know, I kinda feel X when people Y". If you lead with you ("I") when giving facts or feelings people generally feel less under attack and it's easier to find the middle ground.
Most of these work best if you're both relaxed and in a friendly place - you're having a chat here, not a fight. Are there any particular parts of this you think might be more/less useful?
If you want to read through a couple of things I think this is totally relevant to all kinds of relationships:
How to Clash With Love: Some Conflict Resolution Basics. Also, the Southern Poverty Law Centre also has a piece full of different perspectives on
Responding to Everyday Bigotry. Anything in there catch your eye?
*Sadly if your goal is changing someone's ideas, I'd say that's pretty ambitious and you might be in for the long haul - so make sure you're looking after yourself in this and give yourself permission to just not have these conversations beyond a certain point.