If someone came to me after discussing salaries and asked for a pay rise because someone else was getting paid more than them, they'd both likely be looking for other jobs.You should. You get told it's rude to talk about it because your boss doesn't want you to know if you're getting underpaid. Classic stitch up.
Absolutely it's possible. And if you find out that's the case you should also ask for a raise or start looking for a better gig. No sense in letting someone take advantage of you.
Earn your stripes and get paid based on your merits. If you have a case for a pay rise, you need a better argument than 'someone else is getting paid more', regardless of whether they are male or female.
How about reducing the higher paid employee's salary to make them 'equitable'?