All religions have grades or levels of practice. For the beginners (ie. singles), agnihotra is a simple ritual with mantras performed by a single person. Yet, that is only the "preschool" version that prepares the person to understand the authentic agnihotra (on the tantric level), which is performed in a marriage between man and woman.
We respect the beginners version, but it is a bit like child's play when compared with the power of the sexual act.
In any case, all of those old, complicated, and long rituals (like the basic agnihotra) are antiquated and unnecessary today. We are now in the Aquarian era, which brings with it a new level of spiritual work. Today, there are more expedient methods to purify the atmosphere, such as incense and conjurations.
"Do not worry; cultivate the habit of being happy." - Samael Aun Weor