Clearly, Jaskier was neverous. Geralt was too but his wasn't outwardly showing. He knew he had his fate tied to Yennefer and that they would live much longer than Jaskier. He knew this was probably the poorest decision he could ever make for both of them to have any sort of feelings beyond friendship be revealed. In the very least, they could have some time together.
Geralt held no hesitation or fear as he kissed him.