Guys can't always tell when they ejaculate. Semen can come out without an orgasm.
Missing a period is a possible sign of pregnancy.
Unfortunately the home tests are not very reliable. They are really only reliable when they indicate "pregnant." They "default" to "NOT pregnant." False negatives are VERY common. False positives are VERY rare.
You must use your very first morning urine to test, your wake up pee. Follow all directions carefully. Wait until at least two week after you had sex OR when your period is due.