Lucie has been postponing giving him a clear answer for a long time now. She did once say she loves him, but she still wouldn't get into a romantic relationship. He made his feelings clear months, perhaps already half a year ago. After that, they have lived a life where they all could have died a number of times, which typically makes people feel like there's no time to waste. Things like the suspension bridge effect seem to not affect Lucie at all.
With all that, perhaps Cyril just couldn't keep his pants on anymore. It was a stupid thing to do, of course, since he still claims he loves Lucie. It would be a different thing if he was ready to let go of Lucie, since Lucie obviously months later remains full of hesitation about Cyril. Or perhaps Lucie simply isn't mature enough for any kind of romance yet. Doesn't really make a difference. He has been getting too frustrated to cope with it anymore.