The Pipe is correct. It's a matter of staying sensitive to the woman's enjoyment of you. Get her enjoying herself and she'll be juicy and enthusiastic. Start ignoring the one you are with and treating her like a fleshlight and she'll get hurt.
Believe me, appreciation will keep you hard! Expect it, fish for it, be more pleasing! And if it doesn't have any effect, scale it back.
Also, vaginal intercourse alone is statistically unlikely to lead to feminine orgasm. Add more stimuli, a lovely cocktail of pleasures.