For nontheistic pagans and witches, altars are meant to bring a certain energy to a space. Altars don’t have to just honor deities - they can honor an emotion, a certain space, the memory of someone who has passed or the holiday that you’re celebrating. Nontheistic altars achieve this through symbolism, arrangement, nature and magick. You can use a variety of items from gemstones, flowers, herbs and candles to figurines and items that represent your goal.
Also keep in mind that gods and goddesses can be metaphors for human emotion. If you’re looking for something that emphasizes love and fertility, there are many, many gods and goddesses that represent that. A word of caution: I would only use god and goddesses as metaphors if you still have respect and knowledge of their history. Don’t just throw Aphrodite on your altar because you’re wanting something for love and that’s all you really know about her. If that’s the case, I’d simply steer clear of all deity representations.