I feel like at the start of every major international tournament, we always hear stories of managers banning all forms of sexual activity- in fact at the 2018 World Cup, 8 teams had some sort of ban on meeting with partners for sexual activity. But does sexual activity truly impact your performance? Honestly, most likely not.
Current research in this topic goes to show that there is little to no impact sexual activity has on physical performance whatsoever. In fact, some research states that sex before matches can actually improve your resting blood pressure! Additionally for some players, it could provide a mental boost, which can somewhat impact physical performance.
However, it is not uncommon to see drugs, alcohol, and poor sleep coupled with sexual activity. We know that all of those do negatively impact performance. So, if you are going to engage in any sort of activity, be sure to avoid those.
Summary: Sex before training or matches likely has no impact on your performance, so feel free to engage in any activities you want. Just get enough sleep and stay away from drugs and alcohol while you do so.