Cucumbers cross only with other cucumbers, etc. In addition, you would not see the result of any cross until you planted the seed from a fruit produced this year and grew out the resultant fruit in the next generation next year. Remember self-pollenized crops do not cross with anything else. The vine crops are insect pollenized so that crossing might happen ONLY for crosses within the crop and ONLY if you save your seed for next year's crop. In plants that are self pollenized, this is not a problem with bees.
Many plants set more flowers than the plant can develop fruit.
Tomatoes, for example, may produce 15 to 20 flowers in a cluster of blooms but only 5 to 6 of those can develop into a fruit. Extensive vine growth is usually associated with poor blossom set. Anything that creates lush vine conditions usually discourages bloom set. In areas that have had a lot of manure applications or areas where extensive N fertilizers have been applied, there is usually a corresponding decline in the failure of plants to bloom properly and some flower abortion. This is often corrected as the season progresses and some of that excessive fertilization is used up or leached from the plant root zone.